143 Caterpillars (Eupithecia Spp.)
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143 CATERPILLARS (EUPITHECIA SPP.) AS OBLIGATORY AMBUSH PREDATORS: A UNIQUE ADAPTIVE SHIFT IN THE HAWAIIAN ISLANDS S.L. Montgomery Entomology Department Ulliversity of Hawaii at Manoa Honolulu, Hawaii 96822 ,Ambush predation, a previously unreported and bizarre feeding habit for the insect order Lepidoptera, has been discovered among geometrid caterpillars of the genus Eupitheaia in Hawai'i. Certain endemic species consume no plant matter whatsoever; instead, they perch cryptically along leaf edges or plant stems and await passing insects. When prey individuals touch their caudal end, the caterpillars thrust the anterior part of the body backwards, seize the prey, and quickly return to the forward position. The entire strike, illustrated in Fig. 1, lasts about one-tenth of a second. While grasping the prey, sharp tarsal claws and spinelike setae of the thoracic legs often pierce the victim's exoskeleton (see Fig. 2). Unlike the prayingmantid, vision is not utilized; one species is particularly alert in darkness. Prey observed being eaten in the field include calliphorid and dolichopodid flies, a cockroach. a gryllid cricket, a cosmopterygid moth, and an ichneumonid wasp. The geometrid genus Eupitheaia has a worldwide distribution and contains about 1000 described species, with nine described species endemic to the Hawaiian Archipelago. None have been recorded previously as obligatorily predaceous (Balduf, 1939; Clausen, 1940) nor have I found any report of specialized behavioral or structural adaptations for capturing active prey by the larvae of any other moth or butterfly species. Those' few predators recorded among the overwhelmingly plant-feeding Lepidoptera larvae are restricted to browsing on sessile, sedentary or otherwise passive prey with the only partially comparable exception to this statement being the West African lycaenids, which Lamborn (1914) found to crawl toward, drop upon, and eat leafhoppers. The first observation I made of this predatory behavior was of a caterpillar perched on a leaf holding and chewing an 8 mm blow fly (Calliphoridae) in a native forest on the slopes of Hualalai Volcano, Island of Hawai'i. This green caterpillar was taken to the laboratory securely perched along the lobeliad leaf edge on which it was cryptically appressed. After two days, the caterpillar had not eaten 144 any of the leaf, and living drosophilidflies were placed in the vial. When a fly first touched the caterpillar's caudal region, a ready position was assumed in which the forehalf of the body is elevated. The caterpillar then struck and captured the fly (Fig. 1) which was then entirely consumed except for wing fragments. After feeding on three additional flies, the larva pupated and the resulting moth was identified as Eupithecia orichZoris Meyrick. Subsequently, E. oriahZoris has been reared through its complete life cycle in the laboratory, and larvae, were proven predaceous throughQut all four instars. One-day-old larvae (2,3 nun length) regularly caught and fed upon living book-lice (iJiposceZis divinato:ris). Larger caterpillars fed ona diversity of appropriate-sized prey, including DrosophiZa, / mosquitoes and· ants. Those prey too large to be held securely, or causing annoyance, were quickly thrown aside. The larvae do not react to flies moving near their heads and either ignore or attempt to shake off flies that crawl onto their heads or the anterior two-thirds of their bodies. Only prey that touch the rear third of the body, including the anal processes (paraprocts), are struck. Feeding experiments with 27 larvae showed that an average of 43 DrosophiZa of 3 rom length were consumed during the final larval instar,which attained an average body length of 21 mIn. Adult moths fed on small amounts of diluted honey and females deposited 60-120 eggs placed in loose clusters of three to five eggs on the underside of leaves. Very similar obligately predatory behavior has been confirmed for an endemic complex of five other Eupitheaia. E. staurophragma has been found on the Islands of lIawai'i,' Maui, and O'ahu. It differs morphologically from E. orichZorisin its longer anal prolegs and shorter anal processes. Also, the caterpillars are brown and almost without exception are f01,lnd perching on twigs rather than leaves. Similar in morphology to E. staurophragma are three undescribed species from Moloka'i, O'ahu, and Kaua'i. In E.craterias, the caterpillar is'light or brownish green and most frequently perched on leaf petioles. It has been collected on three islands and is distinguished by 'elongate setae on its posterior dorsum. E. scoroiodes has a dark-colored larva and is known only from the subalpine zone of Maui, above 2000 m or 6500 feet elevation. All of these and two additional undescribed species from the Island of O'ahu possess elongate, heavily spined thoracic legs and have been observed to capture and consume prey in a manner near to that of E. orichZorois. Although not fully investigated, E. rhodopyra is most likely a sixth obligatorily predaceous species, since the legs of the first instar are specialized for seizing prey. 145 Most of these carnivorous geometrid larvae are sparsely but widely distributed in native forests and ~hrublands above 300 m in elevation. Over 300 specimens have been collected, usually well camouflaged and stationed within 1 m of the ground in partially shaded situations. They are quite sedentary, remaining in their perch position for days or weeks to snap up approaching insects., The only Hawaiian species of Eupithecia which is principally phytophagous is E. monticoZens. Swezey (1954) reported that it preferred the epacrid, StypheZia, but also fed on plants belonging to five families. I ,have learned that an important food is the protein-rich pollen of the abundant flowers of Metrosideros. E. monticoZens has no leg or other specializations facilitating entomophagy, but cannibalism and necrophagy have been observed in the laboratory. '. The radiation of Hawaiian Eupitheciainto at least six predatory species is an intriguing instance of adaptive shift. These geometrids were probably inclined to predation by diet that included protein-rich pollen; as now seen in E. monticoZens. The behavior of snapping backwards is common to many geometrid larvae and is used in defense. With the Hawaiian Eupithecia, this behavior was intensified and refined into a grasping strike. A significant factor in allowing this behavioral shift in the geometrids was the disharmonic nature of the biotic environment. Of the approximately 250 insect colonizers of Hawai'i, unusually few were predators, with carabids, reduviids, nabids, hemerobiids, and chrysopids predominating (Zimmerman, 1948). Only the emesine assassin bugs, nabid bugs, and spiders were able to establish and radiate in the Hawaiian Archipelago. Analogous groups present on the continents, stich as the ambush bugs, mantids, and mantispids were totally lacking prior to contact by western man. The absence of many predators and parasites that attack lepidopterous larvae would also have permitted Eupithecia to adopt an ambush predation that involves waiting for prey in-relatively exposed sites. Without doubt, these Hawaiian inchworms have one of the most interesting or "striking" life histories of any moth. Being unique to the Hawaiian Archipelago, it is a prime example of the adaptive shifts and radiations so notable in the biology of islands. Because at least four species are readily found within the National Parks in Hawai'i, it is recommended that the "grappling inchworms'" behavior be interpreted at Park visitor centers by film and photographic methods. If such a phenomenon could go undetected in Hawai'i until the 1970's, it prompts one to ask what other rewards await the cU,rious naturalist in the earth's most singular island group. 1. Waits 2. Poises 3.Striltes A.Feeds Figure 1. Diagram of ambush predation by Eupithecia. Figure 2. E. staurophragma poised ready to strike. Figure 3. E. craterias Figure 4. The short-legged completing a meal. E. monticolens is predominately a fruit and flower feeder. 148 Acknowlede;ements The author, grate£ullyac~nowledge~ the assist~nce of numerous colleagues, in particular Wi~liam P. Mull of Volcano, Hawaii, who provided the phot6gr~phs and many detailed observations. The drawing w~s prepared by . David Kemble, courtesy of the Bishop Museum Entomology Department. Moth identifications were' confirmed by Dr. Klaus Sattler who is revising the Ha,wa:Lian Eupithecia at the Britishl1useum (Natural History) in London. This work was supported in part by National ScienceFQundation Grant; GB 29288. .. Literatur~ Cited Balduf, W. V. 1939. The bionomics of entomophagous insects, Pa,rt 2. Swift Co., St. Louis. .. ,. Clausen, C.P. 1940. EntoIDophagous insects. M:cGraw-Hi11, New York. 688pp. Lamborn, W.A. 1914. On the relation~hip between certain W. African insects,. especially ants, Lrcaenidae and Homoptera. Ent. Soc. London,· Trans. '( 913) ~ 436-498. Swezey, O.H. 1959. Forest entomology in Hawaii. Spec. Publ ..B. P. Bishop Museum 44. 255 pp. Zimmerman, E.C. 1948. Insects of Hawaii, Vol. 1, Introduction. Univ. of Hawaii Press, Honolulu..