On Predation by Robber Flies (Diptera : Asilidae)

Current Research in Neuropterology. Proceedings of the Fourth International Symposium on Neuropterology. BagnBres-de-Luchon, France, 1991. Canard, M., Aspiick, H. & Mansell, M.W. (Eds). Toulouse, France, 1992. Pp. 313 - 318.

Influence of hindwing size in nemopterids (Insecta : Neuroptera : Nemopteridae) on predation by robber flies (Diptera : Asilidae)

Mike PICKER l, Belle LEON & Jason G.H. LONDT University of Cape Town, Rondebosch, South Africa University of Cape Town, Observatory, South Africa Natal Museum, Pietermaritzburg, South Africa

ABSTRACT

Two functions of the hypertrophied hindwings of Palmipenna aeoleoprera Picker and P. cf. pilicornis Tjeder were examined : that of decoys in redirecting robber fly attacks, and their contribution to an illusion of apparent greater size. Presentation trials using various lures of the two species of nemopterid with and without hindwings showed that the hindwings did not redirect robber fly attacks to the less vulnerable parts of the body. Instead, they depress robber fly predation. Only the largest class of robber fly attack the lure with the largest hindwings (P. aeoleoptera) to any extent. The somewhat smaller P. cf. pilicornis lure had a similar effect, not as pronounced as the P. aeoleoprera lure. It is thus probable that the hypertrophied hindwings reduce the attack rate by the more numerous, smaller species of robber flies at the study site.

Key words : Nemopteridae, Palmipenna spp., hypertrophied hindwings, Asilidae, Neolophono- tus spp., prey selection, visual deterrence.

INTRODUCTION

The highly modified hindwings of Nemopteridae are unique among the insects, but their function remains enigmatic. The greatly enlarged and richly pigmented hindwings of Palmipenna aeoleoptera Picker are probably the most hypertrophied of all nemopterids, and thus provide an ideal model for investigating the function of the nemopterid hindwing. Previous studies on this species have ruled out an intraspecific semaphore function (PICKER& LEON1990) ; neither are they used in thermoregulation (LEON& PICKER 1990a, 1990b). This study exa- mines their possible antipredator role. They may function in the same way as the M. Picker, B. Laon & J. G.H. Lmdt elongated decoys at the tips of ly~aenidbutterfly hindwings which redirect predators to less vulnerable parts of the body (mu& L~XDALE1988). Alternatively, they may present an illusion of greater size to visually hunting predators such robber flies, which fdextensively on Palrnipenm. Robber flies use size and other parameters of the potential prey such as hardness, wlour and flight speed during the brief aerial assessment of the potential prey item (LMGNE 1984 ; SHELLY1984). The size of the prey is the most important discriminant used by attacking robkr flies (SHELLY & PEARSON1978, 19801, with larger robk flies attacking larger prey items. As the hindwings of P. aeoleopo more than dauble the body s'm, they may well confer an illusion of larger apparent size to attacking robtrer flies. In this study, we compare the response of robber flies to two closely related species of Pahpenna which co-occur in the Biedouw Valley, albeit in different habitats ; P. cf. pilicomk Tjeder is restricted to sandy areas, while P. aeoleoptera is ordy found on stony slopes. The two species are very similar in both body size and form, the major difference king the shape and size of the hindwings (Fig. 1).

Fig. 1. Males of P. ueoIcoprera (top mw) and P. cf. pilicomis (bottom row) showing thr hypertrophied and contraxtingly pigmented hiidwings. The isolated hindwings of each apecim arc shown alongside the intact male. Hindwings of Nemopteridae MATERIAL AND METHODS

The study was carried out in the Biedouw Valley in Namaqualand, South Africa (32"05'S, 19" 15'E) during the brief imaginal emergence in spring. Robber flies observed catching or feeding on prey were collected. Random sweep nettings of the general insect fauna at the study site were used to represent the prey avail- able to robber flies, and this was then used in calculating the feeding selectivity (S) indices (BERG 1979). The score assigned to a prey taxon was then used to determine if the prey item was positively selected, or avoided. We used live tethered lures of P. aeoleoptera and P. cf. pilicornis to test the effect of the hindwings on prey selection by robber flies. In 1990, robber flies were presented with the intact lure of P. aeoleoptera first, and then the lure of the same species with hindwings amputated (PICKER,LEON & LONDT 1991). Where- ver possible, the position of attack on the intact lure was noted. In 1991, other presentation trials used paired combinations of intact P. aeoleoptera and P. cf. pilicornis, P. cf. pilicornis with and without hindwings, and controls comprising both species without hindwings. In all of the latter trials, the order of presentation of the lures was alternated regularly. A response was scored as an attack when the robber flies flew towards, and contacted the lure.

RESULTS

P. aeoleoptera presentations

In 1990, eight species of robber fly were collected from the study site. They fed on a wide variety of prey, with Diptera (31 %), Coleoptera (19 %), and Pal- mipenna (19 %) predominating. Palmipennu aeoleoptera is apparently an important prey item for the robber flies as it scored a positive selectivity index value (PICKER,LEON & LONDT1991). The position of attack by robber flies occurred most frequently on parts of the body other than the hindwings (x2 = 29.2, n = 14, P < 0.001).

Table I. Responses of six species of robber fly to presentations of lures. The lures comprised live P, aeoleoptera with hindwings either intact or amputated.

Asilidae species length (mm) intact amputated not X f SD hindwings hindwings attacked (n) (n) (n)

Neobphonotus abuntius 14.9 f 2.1 1 1 4 N. bimaculatw 14.8 * 1.3 7 3 1 N. expandocolis 12.0 1.5 1 16 17 Acnephalum andrenoides 13.9 0 0 1 Cyrtopogon sp. 7.8 0 0 1 Gonioscelis haemorhous 15.9 0 0 1

totals 9 20 25 M. Picker, B. Leon & J. G.H. Londt

Most (94 %) of the robber flies used in the presentation trials belonged to the genus Neolophonotus Engel, and each of the three predominant Neolophonotus species responded in a different way in the presentation trials (Table I). Neolo- phonotus bimaculatus Londt had the greatest response rate (91 %) to the lures. This species and N. abuntius (Walker) were the largest species of robber flies, and when considered together, attacked the intact lure far more frequently than the smaller N. apandocolis Londt (P < 0.001, 2 X 2 contingency table). The lure with amputated hindwings was attacked more frequently than the intact lure (X2 = 4.16, n = 29, P < 0.05), largely attributable to the response pattern of N. expandocolis. Overall, the mean body length of robber flies attacking the intact lure (14.97 f 2.08 mm, n = 9) was greater than those attacking the lure with amputated hindwings (12.44 f 2.37 mm, n = 20, Mann-Whitney U-test, P = 0.05).

Mixed species presentations

Similar size classes of robber flies attacked both P. aeoleoptera (mean length = 13.7 f 1.54 mm) and P. cf. pilicornis (13.2 f 1.71 mm) when they were used as lures. Far more robber flies tended to attack the smaller P. cf. pilicornis lure (n = 48) than the larger P. aeoleoptera (n = 19). The group of robber flies that did not respond to either lure was significantly smaller (mean length = 12.88 & 1.38 mm ; P = 0.026, t-test) than those that did respond, fur- ther indicating that large intact lures deterred the smaller robber flies. When controls consisting of P. aeoleoptera and P. cf. pilicornis lures without hindwings were presented, the same number (23) of robber flies attacked both lures, and comprised flies of similar size (P = 0.9, t-test). The final pair of lures comprised P. cf, pilicornis with and without hindwings. The latter lure was attacked at a somewhat higher rate than the former (Table 11). The average length of the robber flies in the two groups did not differ significantly (p = 0.91, t-test).

Table 11. Attack rate in response to presentations of the various lures (attack rates are means derived from all trials involving the various lures).

lure attack rate (%)

P. aeoleoptera intact 13 P. cf. pilicornis intact 28 P. cf. pilicornis amputated 34 P,aeoleoptera amputated 37

The attack rate seen here falls off with increasing apparent size of the insects. The hindwings of P. aeoleoptera are larger than those of P. cf. pilicornis resul- ting in a greater apparent size of the former. The percentage of non-responding robber flies in the various presentations also supports this trend. The presentation using intact insects of both species has a high percentage of non-responding flies (55 %). In the presentation using intact and amputated P. aeoleoptera, 43 % of flies did not respond. In similar trials using P. cf. pilicornis instead of P. aeoleoptera, 41 % of the robber flies did not respond. When lures of both species Hindwings of Nemopteridae of lacewing with amputated hindwings were used, only a small percentage of robber flies (26 %) did not respond.

DISCUSSION

Our results suggest that prey selection by robber flies is influenced greatly by the size of the potential prey item. The nemopterid with the largest and most conspicuous hindwings (P. aeoleoptera) elicited the lowest attack rate. Moreover, the robber flies that did attack this nemopterid were, on average, the largest class of robber flies present at the study site. They comprised chiefly N. bimaculatus and N. abuntius. Since the only difference between the two P. aeoleoptera lures used in this presentation was the presencelabsence of hindwings, the observed trend must have been brought about by the hypertrophied and richly pigmented hindwings of P. aeoleoptera that contributed to an illusion of greater size, and depressed the attack rate. They do not, as was initially hypothesized, redirect attacks to the less vulnerable parts of the body, but by creating an illusion of greater size deceive the sensory system of the attacking robber fly. Consequently the nemopterid becomes less prone to attack by the smaller, but more numerous N. apandocolis. It is well-known that larger species of robber flies attack larger prey species (SCARBROUGH1982), and this has been demonstrated experimentally using cardboard models of various sizes as lures (SHELLY& PEARSON1980). The inclusion of P. cf. pilicornis in additional presentation trials served to reinforce the above conclusions. The hindwings of this species are not as large as those of P. aeoleoptera, this being reflected in the larger number of robber flies that attacked the intact P. cf. pilicornis lure when it was presented ir_ trials against the intact P. aeoleoptera lure. The attack rate of robber flies on the intact P. cf. pilicornis lure was depressed somewhat when compared to the amputated lure of the same species. Thus the hindwings of this species reduce the attack rate to a lesser extent than those of P. aeoleoptera. While reducing predation by robber flies by increasing apparent body size, the hindwings of these unusual lacewings probably serve additional functions. Their antipredator role may even be an effect rather than a primary function. They cer- tainly serve an aerodynamic role as gradual removal eventually destabilizes normal flight, causing uncontrolled tumbling. The larger relative size of the hindwings in males, and their often dramatic pigmentation both suggest some additional function. However, observations to date on mating behaviour do not suggest any visual involvement.

REFERENCES

BERG,J. 1979. Discussion of methods of investigating the food of fishes, with reference to a preliminary study of the prey of Gobiusculusfivescens (Gobiidae). Marine Biology 50 : 263-273. LAVIGNE,R.J. 1984. Ethology of Neocerdktus acutangulatus (Diptera : Asilidae) in South Australia. Proceedings of the Entomological Society of Washington 86 : 422-43 1. M. Picker, B. Leon & J. G.H. Londt

LEON,B. & PICKER, M.D. 1990a. Function of the hindwings of Pahipenna aeoleoptera Picker (Insecta : Neuroptera : Nemopteridae). In : Mansell, M.W. & Aspiick, H. (Eds) Ad- vances in Neuropterology. Proceedings of the Third International Symposium on Neu- ropterology. Berg en Dal, Kruger National Park, R.S.A., 1988. 89-94.Pretoria, R.S.A. LEON,B. & PICKER, M.D. 1990b. Behavioural therrnoregulation in Pahipenna aeoleoptera Picker (Neuroptera) : Do the hypertrophied hindwings play a role ? Journal of Insect Be- havior 3 : 381-394. PICKER,M.D. & LEON,B. 1990. Population biology of Palmipenna aeoleoptera Picker with reference to the possible semaphore function of the hindwings (Insecta : Neuroptera : Nemopteridae). In : Mansell, M.W. & Aspiick, H. (Eds) Advances in Neuropterology. Proceedings of the Third International Symposium on Neuropterology. Berg en Dal, Kruger National Park, R.S.A., 1988. 95-99.Pretoria, R.S.A. PICKER,M.D., LEON,B. & LONDT,J.G.H. 1991. The hypertrophied hindwings of Pahipenna aeoleoptera Picker, 1987 (Neuroptera : Nemopteridae) reduce attack by robber flies by increasing apparent body size. Animal Behaviour 42 : 821-825. RILEY,A.M. & LOXDALE,H.D. 1988. Possible adaptive significance of 'tail' structure in 'false head' lycaenid butterflies. Entomologist's Record and Journal of Variation 100 : 59-61. SCARBROUGH,A.G. 1982. Coexistence in two species of Holocephala (Diptera : Asilidae) in a Maryland habitat : predatory behavior. Proceedings of the Entomological Sociely of Wmhington 84 : 349-365. SHELLY,T.E. 1984. Comparative foraging behavior of Neotropical robber flies (Diptera : Asi- lidae). Oecologia 62 : 188-195. SHELLY,T.E. & PEARSON,D.L. 1978. Size and color discrimination of the robber fly Efferia tricella (Diptera : Asilidae) as a predator on tiger beetles (Coleoptera : Cicindelidae). En- vironmental Entomology 7 : 790-793. SHELLY,T.E. & PEARSON,D.L. 1980. Predatory behavior of Proctacanthella leucopogon (Diptera : Asilidae) : Prey recognition and prey records. Environmental Entomology 9 : 7-9.

Addresses of authors :

Dr Mike D. Picker Dr Belle Leon Dr Jason G.H.Londt Zoology Department Department of Physiology Natal Museum University of Cape Town University of Cape Town Pietermaritzburg 3201 Rondebosch 7700 Observatory 7925 Republic of South Africa Republic of South Africa Republic of South Africa Bibliography of the Neuropterida

Bibliography of the Neuropterida Reference number (r#): 7306

Reference Citation: Picker, M. [D.]; Leon, B.; Londt, J. G. H. 1992 [1992.06.??]. Influence of hindwing size in nemopterids (Insecta: Neuroptera: Nemopteridae) on predation by robber flies (Diptera: Asilidae). Pp. 313-318 in Canard, M.; Aspöck, H.; Mansell, M. W. (eds.). Current Research in Neuropterology. Proceedings of the Fourth International Symposium on Neuropterology (24-27 June 1991, Bagnères-de-Luchon, Haute-Garonne, France). Privately printed, Toulouse, France. 414 pp.

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