AN UNUSUAL CASE OF MIMICRY

Abstract

Numerous spiders mimic ants, and considerable descriptive literature is available (Bonnet, 1945); other spiders are supposed to resemble bird droppings or certain insects. On the other hand, literature on animals that mimic spiders is scant and no references could readily be found. Spiders, however, are poisonous to various groups of animals and should present evolutionary opportunities to potential mimics. Spider beetles (Ptinidae) at times are brought in, mistaken by collectors for spiders; their relationship to spiders is obscure. On a trip to Dalmatia, in the vicinity of Dubrovnik, I found the pill bug, Armadillidium klugii Brandt 1833 (Crustacea: Isopoda), to be so similar to Latrodectus mactans tredecimguttatus (Rossi, 1790), the malmignatte or European form of the black widow, as to be easily mistaken for it. The northern Mediterranean widows have bright red spots on a glistening black body (Fig. la), coloration resembling that of juvenile American forms. The spots are usually absent in adults in other parts of the range of L. mactans. The spiders are found among piles of stones, roadbanks, and deep plow furrows. Among stones in the Dubrovnik area we found Armadillidium klugii, a pill bug endemic in this area of Dalmatia and on adjacent islands, that rolls up when disturbed, showing bright red spots on a shiny black background (Fig. lc), strikingly like the abdomen of Latrodectus. The isopod is found commonly in crevices and between rocks. It was described from alcoholic specimens (Strouhal, 1928) in which the red color had washed out leaving yellow spots instead of red. Bright colors are not commonly found among pill bugs, and the coloration thus is most unusual. According to Strouhal, the species is variable in color, black to brown-black, with spots whitish to yellow and telson yellow to dark in alcohol. No black widows were collected with the isopod but this was perhaps due to the extremely low population of L. mactans in summer of 1962, the lowest in at least 10 years both in Italy and Yugoslavia (personal communication, Z. Maretic and M. Alessandrini), possibly due to unusually cold weather late in spring. The area probably normally supports a large black-widow population. There also occurs on the ground between stones in the Dubrovnik area, a pill millipede Glomeris pulchra C. L. Koch 1847 (Diplopoda: Glomerida), a black species with a series of bright red spots. When disturbed the millipede rolls up like a pill bug (Fig. lb), also strikingly resembling the malmignatte abdomen. Dr. P. Tongiorgi made me aware that pill millipedes of the genus Glomeris in northern Italy may have a similar color pattern. Live animals of a different species from Pisa, Italy, were also all black with two series of spots varying in color from yellowish to reddish; none had bright red spots, but no specimens were obtained from areas where Latrodectus is common.' Mimicry depends upon a characteristic of a model that makes it of selective advantage to be confused with the model. Latrodectus is known to be extremely poisonous to some vertebrates-guinea pigs, horses, camels, and man. Its poison must be effective on some invertebrates as under its webs were found scarabid beetles several times the volume of Latrodectus. The nocturnal habit of mammals and their sensitivity to the poisons would preclude any advantage of mimicry as a means of avoiding predation by them. Superficial acquaintance with the habitat suggests that lizards, abundant everywhere, might be the most important predators of isopods, glomerid millipedes, and ground-dwelling spiders. The only reference that could be found (Vellard, 1936) indicates that Latrodectus is poisonous, but not lethal, to lizards. If the spider should succeed in biting a lizard attempting predation, the lizard would have an opportunity to learn not to attack Latrodectus again, and mimicry might very well give a selective advantage against lizard predation. As far as is known (Mertens, personal communication), Mediterranean lizards of the genus Lacerta feed on isopods, and it is believed that the rolling up of Armadillidium would not serve as complete protection. Another possibility is that the mimics display similar aposematic coloration. However, isopods are not known to be distasteful and the defensive secretions of most millipedes are not well-developed in the glomerids; a pair of glands is present but the secretions are odorless and tasteless (Attems, 1926).