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of f/ie Amateur Entomologists' Society
Volume 71 • Number 500 February 201 2
ISSN 0266-836X
Editors: Dr P. Wilkins & Paul Sokoloff
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Bulletin Cover
Bulletin
of the Amateur Entomologists' Society
Volume 71 • Number 500 February 201 2
ISSN 0266-836X Editors: Dr P. Wilkins & Paul Sokoloff
The photograph on this month's cover is the Red and Black Froghopper, Cecropia vulnerata.
This very distinctive species belongs to the Homoptera, (Auchenorrhyncha, family Cercopidae) and is related to the cicadas. The larval stages of froghoppers are most commonly associated with "cuckoo-spit" on the stems of plants, but those of C. vulnerata are rarely seen as they live, often communally, on underground roots, protected by solidified froth .
The adults are quite common, and can be seen in suitable localities from April right through to July. Like all of the froghoppers, it has a well-developed jumping mechanism to escape danger. Jumping movements are poorly understood. This froghopper can jump within one millisecond of a stimulus, which is too fast to be explained by muscle power alone, and suggests an additional "elastic" tension is present within the hind legs. Microscopic examination supports this view as an elastic energy storing molecule, called resilin can be found in small structures on the metathorax. One interesting issue is how the froghopper synchronises its two legs for such a rapid jump. One can imagine the consequences of the legs firing out of sequence!
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of the Amateur Entomologists' Society
Volume 71 • Number 500
February 2012
Editorial
As the entomological year stirs, the editor reflects on the inadvisability of making predictions about the weather and its consequences. We remember our very own “Michael Fish moment” last year when an editorial was written during the prolonged spring drought - at least in the southern parts of the country - but by the time the Bulletin was published there was torrential rain. Nevertheless, the absence of early rain, the warm spring and other factors did seem to influence our native insects, with moth trap operators noticing the early arrival of many species, some many weeks before their normal emergence time.
In the February and June editorials of the Bulletin we drew attention to the debate on the government’s proposals on the future of woodlands in this country, and reported on the setting up of an Independent Panel on Forestry. This Panel invited public comment and over 42,000 responses were received. The Panel issued its first report in December 2011 and the initial conclusion that it is important to retain a public forest estate is a welcome one, as is its stated desire to increase woodland cover and access to woodland, and its recognition of the need to restore damaged ancient woods. Despite this the report fails to make a commitment to review the strength of woodland protection. This is critical as it becomes increasingly clear that existing levels of protection cannot be guaranteed into the future. We have such a low level of woodland cover with so many pressures on land; we cannot afford to be complacent about its protection. You can access the report through the DEFRA website on: http://www.defra.gov.uk/forestrypanel/reports/
In recent editorials both of the editors have appealed to members for contributions to the pages of the Bulletin and I am very grateful to those members have responded to this appeal. We want to fill six issues a year with material that is readable and of interest to members, so please keep the contributions coming!
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Paul Sokoloff
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Society Matters
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The following members will retire from the AES Council by rotation at the AGM: Peter Hodge; John Howells; Wayne Jarvis; Dr David Lonsdale; Dr Kieren Pitts. Of these, Peter Hodge, Wayne Jarvis, David Lonsdale and Kieren Pitts have expressed their willingness to remain on Council if duly nominated and elected. Mr Ralph Hobbs was co-opted to Council in December and will therefore also stand for election at the AGM.
Our President, Mr Peter Hodge, has agreed to accept Council’s nomination to remain as AES President for the further period April 2011 - April 2012. Additional nominations for Council membership and positions are invited in advance of the AGM or on the day. Members
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wishing to find out what Council membership involves should contact the Hon. Secretaiy or any Council member.
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WAfURAl
HISTORY MUSEUM
0 6 MAR 2012
PRESENTED ENTOM LIBRARY
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A study of gregariousness — larvae of the Buff-tip Phalera bucephala (L.) (Lep.: Notodontidae), and similarities with Thaumetopoeidae
by Martin Probert (1 40 71 )
55 Higher Compton Road, Hartley, Plymouth PL3 5JA.
Abstract
Observations of groups of larvae of Phalera bucephala , from first to final instar, were made in each of three successive years, and details recorded of the feeding-and-resting cycle and its changing nature, of the effect of leaf size on behaviour, of the strength of the instinct to remain together, of the splitting-up and reconnection of sub-groups of larvae, of the massed migration of larvae from one part of a tree to another, of similarities with behaviour of processionary caterpillars in the family Thaumetopoeidae, of a redundant behaviour in the later instars, and of the gradual breaking-up of the original company.
Keywords: Lepidoptera, Notodontidae, Phalera bucephala , larvae, gregarious, processionary, Thaumetopoeidae.
Introduction
Concerning the larva of Phalera bucephala , Richard South tells us that ‘It feeds in companies . . . until nearly full grown’ {Moths of the British Isles , 1961).
The Rev J. G. Wood gives more detail: ‘The eggs . . . are laid in batches, sometimes as many as sixty in number . . . and when hatched, the little caterpillars belonging to each brood remain together, and feed upon the upper surface of the leaf. After their first change of skin, they break up into six or seven small companies, and each company remains together until the change into the pupal state is at hand. As they become larger they make their way to the topmost branches . . .’ (. Insects at Home, 1883).
These details accord with my own observations, but there is much of interest to add.
In 2009 I encountered a group of 70 first-instar larvae feeding on a five- metre high specimen of a non-native species of Hazel called Filbert 0 Corylus maxima ) in my garden. I observed these feral larvae several times a day until they pupated. Surprisingly not one was lost to either wasp or bird, although a few succumbed to disease. The following year. 2010, I introduced further first-instar larvae to the same tree, and again
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observed them daily for two months until they left the tree to pupate. I retained a few pupae, kept them over winter, obtained some moths, then eggs, and in 2011 the resultant first-instar larvae were bred indoors on Rose {Rosa) and Grey Willow ( Salix cinerea ), plants of which the leaf size is considerably smaller than that of the Filbert. One group of first- instar larvae were introduced to a leaf on the same Filbert as in earlier years, but these rapidly diminished in number. There were just five remaining on the leaf when I had the (mis)fortune one morning to see a wasp reduce their numbers to nil in a matter of seconds. When the larvae indoors had reached the third instar, the majority were moved outside to the Filbert and successfully completed their development there. Except for the experiments made indoors on Rose and Willow, the bulk of the following observations are of larvae ranging freely over the Filbert.
The larvae are hairy: ‘downy’ is how W. F. Kirby describes them (. British Butterflies, Moths and Beetles , 1887). The usual precautions were taken (minimal handling, not rubbing the eyes, washing the hands) and no irritant effects were experienced.
Jim Porter mentions that the larvae ‘are very easy to rear in confinement’ {Colour Identification Guide to Caterpillars of the British Isles , 2010), to which I would add that the later instars, when kept in captivity, exhibit atypical behaviour. To express a full range of behaviour, the larvae require more than food: they require space.
First-instar larvae
The first-instar larvae of P. bucephala station themselves side by side on the upper surface of a leaf, nibbling away at the upper layer (Figure 1), consuming the upper epidermis and the central (green and nutritious) mesophyll, and leaving the (translucent) lower epidermis. The lower epidermis, as seen from above, is divided by a neat arc from the untouched part of the leaf. The feeding position shown in the photograph, and the cycle of activity to be described in this section, is unique to the first instar. After about five minutes, the larvae retire in ones and twos and groups to the edge of the leaf, slink over the edge, and gather together on the lower surface. Here, sheltered from rain and sun and overhead predators, and with their frass falling freely to the ground, they rest for perhaps 20 minutes. Then, the time of rest over, the group becomes active once more. Each larva hurries back to the edge, reappears on the upper surface, and gallops for position at the previously nibbled arc. As soon as it arrives, feeding recommences as the larva continues to gnaw away at the upper layer. The whole group are soon lined up again neatly side by side. Late comers may find themselves confronted by a
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wall of tails, so are forced to wander back and forth, seeking an opening. Having found a gap among the line of tails, they work their way in until their body is in line with those of the adjacent larvae. The fresh bout of nibbling will continue for five minutes. The entire cycle of upper-surface feeding and lower-surface resting will repeat, over and over and over again, for several days.
Figure 1. First instar larvae feeding from upper surface. Photo: Martin Probert
Presumably, from the point of view of nourishment, it would make little difference to the larvae if, rather than consume the upper epidermis and mesophyll, they fed off the lower epidermis and mesophyll. But, in moving onto the upper surface as they do, the larvae place themselves in a position which they otherwise tend to avoid. What advantage is gained by this behaviour? One possibility is that upper-surface feeding leaves the lower surface in a pristine condition, offering the larvae, when resting upside-down beneath the leaf, a secure foothold and a firm anchorage for silk life-lines. Lower-surface feeding, by removing the lower epidermis and central mesophyll, would leave the larvae hanging upside-down beneath a chewed underside of upper epidermis. Such a surface, sullied perhaps with loose particles of mesophyll, might be a risky place for gregarious larvae to gather.
Attacking a fresh leaf
When first starting to feed upon a leaf, the larvae, from their preferred position on the lower surface, will advance to the edge of the leaf, bend
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their heads around onto the upper surface, and begin nibbling the upper layer (Figure 2). With time, as the nibbled arc retreats from the leaf edge, the larvae will be forced to move more and more onto the upper surface.
This initial position can be observed more than once when a large group feeds upon a plant with small leaves. A small leaf size necessitates a move to a fresh leaf when the upper layer of the first leaf is consumed.
Figure 2 First instar larvae attacking a fresh leaf. Photo: Martin Probert
The effect of leaf size
On a large leaf, such as Filbert, the arc at which the larvae line up grows longer with each bout of feeding. But on a smaller leaf, such as Grey Willow, the opposite may occur. As the nibbled area spreads over a small leaf, the arc at which the larvae line up will initially grow longer, but eventually will grow shorter. As a consequence, fewer and fewer larvae are able to feed simultaneously. An interesting situation results. The group splits into two, with off-set feeding times. One group feeds while the other rests. When the feeding group starts to break up, the other group starts to wake up, and the two groups change places, those from the first group struggling to back out against the advancing larvae of the second group, while those of the second struggle to find an opening through the retreating members of the first. Both groups remain on the upper surface, exchanging places, and the surface becomes sprinkled with minute pellets of frass.
It may be that the number of larvae I introduced to the leaf of Grey Willow exceeded the number of eggs that would be placed on such a leaf in the wild. (If the number introduced was indeed excessive, then
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the way the larvae adapted themselves to the novel situation is noteworthy.) I have twice discovered first-instar Buff-tip larvae on Grey Willow, but, regrettably, made no record of the number. It might be interesting to accumulate records of the number of first-instar larvae discovered on leaves of different size.
Timing the feeding cycle
It is difficult to fix a precise moment at which to time the duration of the feeding-and-resting cycle. During the feeding phase, when the larvae are on the upper surface, they are easy to see, so a point in this phase was chosen for timing the cycle. The point chosen was that at which, after having fed, roughly half the larvae had left the line.
The most striking observation was that of a group on a Rose leaf. The times at which half the larvae had departed were recorded as 15:11, 15:40, 16:08, 16:36 and 17:04. The durations of each cycle were thus 29 mins, 28 mins, 28 mins and 28 mins. A similar constancy was recorded during other observations: 27, 34, 27, 20, 28, 23, 20 (a different group on Rose); 28, 23, 23 (a group on Grey Willow); 36, 34, 28 (a different group on Grey Willow). All these observations were of larvae kept under cover. Outdoors, the group of 70 on Filbert took roughly an hour to complete each cycle. The differences in the average lengths of these cycles may depend upon factors such as the age of the first-instar larvae, the digestibility of the leaf, and temperature.
Given a particular group of larvae under observation, it is possible with reasonable accuracy to predict the time of the next feeding phase. I made good use of this fact many times while observing various groups of larvae, taking advantage of the known duration of the resting phase to pursue other activities, while being on hand to witness the next bout of feeding.
Larvae of the second and later instars
When first-instar larvae are about a week old, and 5mm long, the first moult occurs. The second-instar larvae remain gregarious, but now consume the entire thickness of the leaf. As there is no longer any necessity to move onto the upper surface in order to feed, the larvae remain for preference on the lower surface, their heads at the edge of the leaf, their bodies more or less side by side, and chew away (Figure 3). Some may rest, stepping back from the edge to do so, while others continue to eat. When a leaf is so reduced that there is little space for all the larvae, half the larva move onto the upper surface. When this happens, the heads of both parties (the upper-surface group and the lower-surface group) are often seen nibbling away at the same section of
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edge. As the leaf is reduced still further, and nothing but a skeleton of veins remains, a search for fresh nourishment will begin.
Figure 3 Second instar larvae feeding from edge. Photo: Martin Probert
Figure 4 Which instar? Illustration from 'South'
Interpretation of an illustration in ‘South’
Plate 40 in Richard South’s Moths of the British Isles shows the larvae of P. bucephala distributed on each side of the midrib of a leaf blade, half
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on the left, half on the right, each larval tail touching the midrib, the bodies parallel to the lateral veins, and with the heads near to, or at, the edge of the leaf. The overall layout of the larval bodies resembles a feather (Figure 4).
From the observations in the preceding sections, we are in a position to identify the instar depicted in South’s illustration. The larvae are not first-instar larvae engaged in feeding, for such larva will always be working away from an edge of the leaf (Figure 1), even when the tail end of the body is on the lower surface (Figure 2). Nor are they first- instar larvae at rest on the lower surface, for, with no food available on the lower surface, the bodies of the larvae are invariably orientated at random. The illustration is of the second (or later) instar. Six of the larvae are feeding, their heads at the edge of the leaf (compare with those feeding in our Figure 3), while the remaining eight, having backed away from the edge, are resting. (And yet, inexplicably, none of the leaf has been consumed!)
As a further reason for these larvae being other than first instar, we note the large heads and comparatively small bodies which are typical of larvae which have recently moulted.
Figure 5 Moult from 4th to 5th instar. Photo: Martin Probert
South’s illustration appears again on page 80 in The Observer’s Book of Larger Moths where the text informs us that ‘the larvae ... eat the lower side only of one leaf’. This is at variance with my observations: the first instar of P. bucephala only ever fed from the upper layer, while the later
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instars consumed the entire thickness of the leaf. The companion volume, The Observer’s Book of Caterpillars, states that ‘when [the larvae] are not feeding they cluster together on a twig where they resemble a dead leaf. This too is something I have not observed. Throughout the three years of the study the larvae have always rested gregariously beneath a leaf. However, when a leaf has been reduced to a near skeleton of midrib and lateral veins, the larvae remain bunched about the veins till the last vestige of edible membrane has gone, and it is perhaps this moment which is alluded to. The larvae may be found in such a configuration either eating, resting, or (see Figure 5) moulting.
Strength of instinct
The motivation to remain together - what I will call the ‘gregariousness instinct’ - is very strong. The larvae move along contiguous joined structures, travelling from leaf to petiole (leaf stalk) to stem, but are extremely reluctant to pass between parts of the plant in incidental contact. To test this observation, I conducted the following experiment.
I cut off a 10cm length of thin stem. The fragment bore several skeletonised leaves and, on one of them, a colony of third-instar larvae. There was no edible membrane on the fragment. This fragment was carefully lodged among the leaves of a quite different stem from the same tree.
The larvae were very active, wandering along the fragment of stem, and up and down the petioles and midribs of the skeletonised leaves. Several of these midribs lay in contact with the surface of fresh leaves of the new stem. But, for three hours, although the larvae never ceased to explore, and constantly brushed against the new leaves, they never once moved en masse from the old stem to the new. Their wanderings remained along the contiguous joined structures of the old stem. A couple of times one individual strayed onto an adjacent fresh leaf, but soon returned to the company of the others.
Eventually, from a position on a skeletonised leaf where several larvae were gathered, and from where it was impossible for those at the edge to retreat because of the presence of those behind, a pair moved onto a fresh leaf. The foremost, with the other in contact behind, moved along the midrib to the tip of the new leaf. Slowly, one by one, others followed.
The numbers on this fresh leaf steadily increased. And yet, two hours later, half of the original colony was still on the original fragment. Any one of these could have reached over and fed from an adjacent fresh leaf without leaving the old stem, but none did so. The instinct to remain together was clearly stronger than the urge to feed in isolation.
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Although this experiment was with captive larvae, I have observed, with groups of larvae living freely on the Filbert tree, this same reluctance to stray onto any part of the tree in incidental contact.
It should be added that a single larva in isolation, such as one moved to a fresh plant, or an individual late in the final instar, is not motivated by the same instinct as one that has just left a group of its fellows, and so may well behave differently. Isolated larvae may move to a part of the plant in incidental contact.
A benefit, and possible disadvantage, of gregarious behaviour
E. B. Ford, in Moths , states that the larva of the Buff-tip ‘apparently produces a rank, disagreeable smell detectable to some people, though not to myself.’ Ford also suggests that an unpleasant odour ‘will be accentuated if the larvae live together in close company’. Such an odour, arising from a company of larvae, may deter one or more of those insectivorous vertebrates capable of climbing, among them some species of mice, shrews, squirrels, lizards, and snakes. Neither odour, nor the covering of hairs, was any protection to first-instar larvae against predation by wasps. Perhaps, in the latter case, the odour acted as an attractant.
Sub-groups, reconnection, and migration
From time to time a few larvae, engaged in feeding on a leaf, will wander off down the petiole to the branch. Here they potter up and
Figure 6 Procession of three larvae. Photo: Martin Probert
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down, slowly, hesitatingly, singly, or in pairs or trios (Figure 6) head to tail, meeting other larvae engaged upon a similar activity, moving a short distance along the branch, then returning to where they started. Where two or more larvae are involved, those following may advance with the head touching the tail of the larva in front, or alternatively may lag behind by some 5cm. In either case, the larvae that follow proceed with confidence, following the path taken by those in front without hesitation.
As time goes on, and the larvae lengthen and grow fatter, and fewer fit comfortably on the same leaf, a group will break up into two or more sub-groups. But, although sub-groups will be found feeding on separate leaves of the same or associated branches, the sub-groups will invariably come together again before moving off in a group to seek fresh food elsewhere. The larvae, after the move, may need to be sought for, having migrated to a quite different branch in another, usually higher, part of the tree. This splitting apart of a group, and the subsequent reconnection of the resultant sub-groups, is a frequent occurrence. The numbers of larvae in groups before and after migration, recorded on several occasions during the observations, were invariably identical: none had taken a wrong turning.
As an example of this reconnection of sub-groups before migration, one morning at 8am there were, on a leaf, eight larvae of the fourth instar of which one was noticeably smaller, and, on a leaf of an associated branch, five larvae of which one again was noticeably smaller. At 6pm the larvae were discovered (by standing on a ladder and searching the upper branches) about 150cm higher up the tree. Eight were on a single leaf, three on the next leaf but one, and two on the next leaf, a total of thirteen larvae, of which two were noticeably smaller, the same totals as at 8am. Each of the two morning sub-groups had descended 50cm till they had come to a common fork, had then descended an additional 50cm, ignoring two forks on the way, changed direction at the third fork, and had then ascended 250cm to reach the evening position. The ascent negotiated six forks, and at each fork each larva had taken the same branch.
Patience ( considerable patience!) will reveal the process of migration from one part of the tree to another. The larvae, after a period of assembly on a branch, will set off purposefully downwards till the branch meets a more substantial branch or the trunk, then climb upwards at a good pace three or four or five abreast (Figure 7), and terminate on one or more leaves at the end of a fresh branch. The sight of scores of purposefully migrating Buff-tips is impressive.
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Figure 7 Massed migration to a higher branch. Photo: Martin Probert
The processionary behaviour of P. bucephala
I have not seen, in accounts of the habits of P. bucephala, any mention of their movements in single file (Figure 6), nor of their massed migrations (Figure 7). Both events are easily missed, and not likely to be witnessed except through lengthy observation. And yet these events may justify us in claiming that Britain has always had, among the native lepidopteran larvae, a processionary species.
The formation in single file of small groups of P. bucephala resembles the lines in single file of the Pine Processionary Thaumetopoea pityocampa (which I once encountered on an alpine plateau in the Canton of Ticino, Switzerland). The gregarious purposefulness of the massed migration of large numbers of the Buff-tip is also reminiscent of that of T. pityocampa. The width of the columns of large numbers of Buff- tips on the move, up to five abreast, reminds me of a description of the Oak Processionary T. processionea, which is said to move in a ‘broader’ procession than T. pityocampa (Chinery 1986).
In his discussion of the Pine Processionary, Fabre mentions that the species is not only encountered in the well-known lines of several hundred
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individuals, but also in mini-processions, sometimes ‘only two in a row’, and such rows of two or three is exactly what we see in the Buff-tip.
The Pine and Oak Processionary, and the Buff-tip, belong to the superfamily Noctuoidea, but, whereas the Buff-tip is placed in the family Notodontidae, the processionaries are today separated from them in the family Thaumetopoeidae. I suspect that, purely on the basis of the similarity of the movements of their larvae, the Buff-tip and the Processionaries may at some stage have been more intimately related on the evolutionary scale than is implied by their present placement in separate families.
The ability of Buff-tip larvae to retrace their steps
A single larva will leave a group and wander off down the petiole, along a branch, proceeding slowly, pausing all the way, turn a fork and proceed along another branch, wander possibly onto a leaf, and then return. One afternoon, studying a group of fourth-instar larvae gathered beneath a leaf, I observed how they were being abandoned and rejoined by lone larvae on a continuous basis. No matter how many forks were negotiated on the outward journey, each larva, in returning, took the correct path at every branching of the way. None failed to return. This same activity occurs well into the fifth and final instar.
It is possible that the means by which the Buff-tip retraces its steps, and the means by which small lines of Buff-tips follow one another along branches, is the same as that described by Fabre in relation to the Pine Processionary. Each larva of the latter species, as it advances, lays a thread of silk. The thread, says Fabre, ‘is so tiny that the eye, though armed with a magnifying-glass, suspects it rather than sees it’. But, in the case of the Pine Processionary, each caterpillar adds to the thread of silk so that, when many caterpillars have passed, and many threads have been added, the trail is easy to see. But whether there are many threads, or just a single thread, it is sufficient for the Pine Processionary to find its way back to where it started.
I was reading Fabre’s description about a month after the 2011 Buff-tip larvae had gone down into the soil. The month had been wet, the rain falling in torrents on many occasions, and it seemed unlikely that any trace of silken threads, if that was how the larvae were retracing their steps, would remain. But, using a xlO magnifier to search those places where the larvae had once been, I discovered, here and there at the tips of shoots, pale threads several centimetres long running parallel to the axis of the shoot. So it would seem possible that the larvae are using the same technique as the Pine Processionaiy to retrace the path. It would
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Bulletin of the Amateur Entomologists' Society
be instructive, when the Buff-tips are active, to examine a branch immediately after the passage of a larva.
I have observed that, as a Buff-tip larva wanders off by itself along a branch, it continuously dips its head towards the surface of the bark. It’s not much of a movement, and the movement is easily overlooked. Closer observation also revealed that, from time to time, the head jerks upward. It is possible that, in making these movements, the larva was either laying out a line of silk, or feeling for the presence of such a line.
A redundant behaviour?
The same jerking movement of the head, described in the previous paragraph, can be seen when larvae are resting together beneath a leaf. In observing this movement we are perhaps witnessing the moment when a larva, anticipating the possibility of a sudden gust of wind, anchors its silk life-line to the lower surface. We shall assume (pending further observation) that this interpretation is correct. This anchoring of a life- line, invaluable to a lightweight first-instar larva, is also carried out by middleweight fourth-instar larvae. When disturbed, lightweight larvae will hang from a thread and climb back up, but middleweight larvae drop to the ground. I suspect that middleweight larvae don’t readily abandon the leaf, for that would be against the ‘gregariousness instinct’. Presumably the life-lines of these middleweight larvae snap, or the larvae are too heavy to make the ascent. So this apparent anchoring of a life-line by middleweight larvae would appear to be what I have called a ‘redundant behaviour’: while there had been an evolutionary advantage to developing this safety mechanism for the benefit of the first instar, there had been no significant evolutionary advantage to losing the behaviour by the time of the fourth instar, and so the fourth instar attaches its line, but gains nothing by doing so.
The breaking-up of the original group
As the Rev J. G. Wood observed, ‘[the original company of larvae] break up into six or seven small companies’. The breaking-up does not, however, occur immediately ‘after their first change of skin’, but may be observed to take place gradually over several instars. Could the break-up result from some deliberate factor? If so, that factor would be in conflict with the ‘gregariousness instinct’. A tendency to occasionally lose the way would seem to provide a simpler mechanism for the partial breaking-up of the original company. Such a tendency may have survival value, enabling the larvae to distribute themselves throughout a tree while remaining in small groups.
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Figure 8 Mature larva (late 5th instar) feeding alone. Photo: Martin Probert
In about two months the impressively large larvae (up to 60mm long) will be in the last days of the fifth and final instar. If on a tree out of doors, they will be feeding on the upper branches (Figure 8), and their eating activity will be clearly visible (given a suitable vantage point) as