larval morphology and searching efficiency in aphidophagous syrphid larvae

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  • Entomol. exp. appl. 43: 49-54, 1987 49 9 Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands

    Larval morphology and searching efficiency in aphidophagous syrphid larvae

    Graham E. Rotheray Royal Museum of Scotland, Chambers Street, Edinburgh, EH1 1JF, UK

    Keywords: larval morphology, locomotive efficiency, capture efficiency, plant substrates, Epistrophe eligans, Metasyrphus luniger, syrphid

    Abstract

    The effects of larval morphology and substrate quality on the searching efficiency of third stage larvae of Epistrophe eligans (Harris) and Metasyrphus luniger (Meigen) were examined. E. eligans has a smooth, flat undersurface and was able to grasp smooth, flat substrates where it achieved high casting rates and cap- ture efficiencies. It is probably best suited to exploiting leaf-feeding, rather than stem-feeding aphids. Howev- er, M. luniger did best on stems and petioles where it used a special U-shaped grasping organ and lateral movements to hold on and move. On these substrates, as opposed to smooth, flat surfaces, it achieved high casting rates and capture efficiencies.

    Introduction

    One of the barriers to the colonisation of plants by insects is attachment or holding on (Southwood, 1973). Many aphid predators actively hunt for prey on plants but how their searching efficiency is af- fected by the attachment barrier is little under- stood.

    Attachment and locomotion are influenced by plant surface qualities. For instance leaf waxs (Ar- zet, 1973; Shah, 1982), hairs (Banks, 1957) and the smoothness of leaves (Carter et al., 1984) interfere with the movements of aphid predators and may cause them to fall off the plant. Coccinellids (Banks, 1957), chrysopids (Arzet, 1973) and an- thocorids (Evans, 1976) preferentially search for aphids along leaf veins and margins but it is not known if this is because attachment is only effec- tive at these sites.

    Another important factor affecting locomotive efficiency is predator morphology such as the na- ture of the locomotive organs and body shape (Giller, 1982). Although predaceous syrphid larvae lack the legs and anal claspers of many other aphidophagous insects, locomotive prominences

    may be present (Heiss, 1938; Bhatia, 1939). These structures vary in form and degree of development among different species. For example, at one ex- treme, they are lacking in the larva of Epistrophe eligans and at the other extreme, are large and com- plex in the larva of Metasyrphus luniger. This pa- per considers the effects on searching efficiency of these two morphological varients.

    Materials and methods

    Larvae were obtained from eggs laid by gravid fe- males caught in the field and placed in Petri dishes with Sitobion fragariae (Walker) aphids on Rubus fruticosus L. leaves. Larvae were subsequently reared on this aphid. Both E. eligans and M. lu- niger feed on S. fragariae in nature.

    The morphology of the undersurface of third stage E. eligans and M. luniger larvae was exam- ined with a dissecting microscope. At least 15 larvae of each species were examined.

    The movements of third stage larvae on Acer pseudoplatanus L. leaves and petioles were record- ed to determine the locomotory mechanisms used

  • 50

    by each species. These leaves have smooth surfaces except for five raised veins on the undersurface of the lamina. A A. pseudoplatanus leaf (approximate size-lamina area = 118 cm2; petiole 9 cm long 2-3.5 mm in diameter) was clamped in ap- proximately parallel position to the horizontal and diffusely lit from above. Observations began when a larva was placed at the base of the petiole. If a larva fell off the leaf it was replaced by a fresh in- dividual until nine larvae of each species had been observed continuously for 1 h. Prior to each repli- cate, each larva was starved for 24 h after initially feeding them with aphids until they were consis- tently refused.

    To catch aphids, syrphid larvae raise the anterior part of the body and expand it forwards or side- ways in a characteristic movement referred to as casting (Chandler, 1969; Rotheray, 1983). The sig- nificance of casting in evaluating locomotive effi- ciency is that without a firm grasp of the substrate with the posterior part of the body, casting may be inhibited or the larva may fall from the plant. Sites where casting took place and the number of casts made during 5 mins following a 2 min initial peri- od on the petiole and the upper and lower surfaces of the lamina were, therefore, recorded.

    Differences in locomotive efficiency on the peti- ole and lamina may influence the effectiveness with which prey are captured. To investigate this possi- bility, 24 h starved, third stage larvae were placed at the base of R. fruticosus leaves (lamina area ap- proximately 35 cm z) or sections of stem (approxi- mately 9 cm long 4 to 6 mm diameter) on which 20 early stage S. fragariae aphids had been placed previously. The R. fruticosus leaves used were smooth with a single raised vein on the undersur- face. Thorns were removed from the plant material prior to being clamped in an approximately parallel position to the horizontal. If a larva fell from the leaf or stem it was replaced by a fresh individual until nine larvae of each species had been observed continuously for 90 min. A larva was introduced only when all the aphids had settled and were feed- ing. The upper and lower surfaces of the lamina were tested separately. The numbers of casts and aphid captures were recorded. Captured aphids were replaced with fresh individuals so that 20 aphids were always present.

    Results

    Morphology of the ventral surface in the third stage larva. E. eligans. The undersurface is smooth, shiny and without raised locomotory prominences. However, along each side of the first seven abdomi- nal segments are discs defined by five to eight shal- low grooves which radiate out from a central point. A network of similar grooves occurs round the tip of the anal lobe at the end of the abdomen (Fig. la).

    M. luniger. The conspicuous feature of the un- dersurface in this species are the raised locomotory prominences. There are seven pairs along the sides of the abdomen and they increase in complexity and size towards the end of the body. They are bi- lobed on segment 1, tri-lobed on segments 2 to 4 and divided into four lobes on segments 5 to 7 (Fig. lb). The tip of each lobe on segments 1 to 4 are coated with numerous flattened spicules. On segments 5 to 7 the tips are smooth, without spic- ules and have grooves radiating from a central point.

    The locomotory prominences on segments 5 and 6 are bent over so that their tips project backwards. The tip of the anal lobe is long, smooth and co- vered with a network of grooves. It forms a 'U' shaped grasping organ with the locomotory promi- nences on segments 5 and 6 (Fig. lc).

    Locomotory mechanisms on A. pseudoplatanus leaves. E. eligans. On the petiole forward move- ments were slow and interrupted frequently by pauses during which the larva either smeared saliva on the petiole or made minor alterations to its posi- tion. Forward movements were made with the body parallel to the longitudinal axis of the petiole and were similar to those of the larva of Syrphus ribesii (L.), i.e. the posterior end of the body is raised and a wave of contraction passes forward to the head (Roberts, 1971). On the lamina, all regions were searched and the raised leaf veins were crossed over without the larva falling off. Significantly fewer pauses occurred on the lamina than the petiole (mean number of pauses on the undersurface of the lamina = 5.8 ___ 2.3 v the petiole = 11.2 + 3.3 in a 17 min period on each site, t = 4.4, P < 0.002). Sig- nificantly fewer casts were made on the petiole than the lamina (Table 1). There was no significant difference between the number of casts made on

  • a 8

    7

    3 4 5

    51

    l 0 3 4 5

    Fig. la-c. (a) the undersurface of the third stage larva of Epistrophe eligans; (b) the undersurface of the third stage larva of Metasyr- phus luniger, actual length = 11 mm; (c) U-shaped grasping organ of M. luniger, lateral view.

    the upper and lower surfaces of the lamina (Ta- ble 1).

    M. luniger. Movements on the petiole occurred with few pauses. In contrast to E. eligans, the larva

    of M. luniger moved sideways along the petiole. This was achieved by curling round the petiole in a semi-spiral and using the 'U' shaped grasping or- gan and mouthparts to hold on. A movement cycle

  • 52

    Table 1. Mean number (_+s.d.) of casts per 5 min period by third stage larvae ofE. eligans and M. luniger on the petiole and upper and lower surface of A. pseudoplatanus leaves.

    Species Leaf

    Petiole Upper Lower surface surface

    E. eligans 4.7-+ 2.6a* 19.9+ 2.0b 20.2+ 3.8b n=9

    M. luniger 38.0+ 4.3a* 12.6+ 2.9b 36.0_+ 5.6a n=9

    * Values in rows are significantly different at P

  • when the larva moves forward, coats the undersur- face and may help to maximise attachment.

    In contrast, the grasping organ and narrow, sub- cylindrical shape of M. luniger allows larvae of this species to curl round stems and petioles and grasp raised leaf veins. On top of the lamina, the larva of M. luniger seems unable to use its grasping organ. This is probably because of the backwardly direct- ed locomotory prominences on segments 5 and 6 which not being in contact with the substrate, result in a reduction of grasping ability. This could ac- count for the unsteady rocking motion and fre- quent falls from the lamina. A minimal grasp of the lamina is further indicated by larvae which al- ways fell off when the leaf was turned upside- down. E. eligans larvae, however, never fell off when the leaf was turned upside-down.

    These differences in

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