Mfi. R. I. POCOCK— MIMICRY IN SPIDERS. 267 



became o£ the thousands of spiderlings. An acquaintance with Trypoxylon 

 has shown us their fate, and has given us an ilhistration of how closely the 

 two groups are related. To make a very modest estimate there must have 

 been twenty wasps at work in our straw stack. During the six weeks which 

 make the busiest part of their working season each of these must have stored, 

 at the very least, thirty cells, putting an average of ten spiders into each cell. 

 It may then be considered certain that the straw stack was the mausoleum of 

 six thousand spiders, and it is very probable that twice as many were interred 

 within its depths. It must be remembered, too, that before the spiders have 

 grown large enough to be interesting to Trypoxylon rubrocmctuni, T. hiden- 

 tatum has had her turn at them, and that those that are allowed to grow too 

 large for T. rubroGinctum are preyed upon grade after grade by T. alho- 

 pilosum and finally by Pelopceics, Pompilus, and other genera.^' The con- 

 clusions embodied in this passage were based, be it noted, upon observations 

 made in a small and circumscribed area in temperate North America. It is 

 justifiable to conclude that persecution at least as dire and destructive in its 

 effects has been carried on season after season probably for many thousand 

 years in all tropical and temperate countries where Pompilidse and Spiders 

 occur ; and setting aside the frigid zones of the world^ the distribution of the 

 two groups is practically cosmopolitan. This being so, it seems to be abso- 

 lutely certain that wasp-persecution has had perhaps a greater effect in 

 moulding and developing the structure and protective instincts in spiders 

 than any other factor in organic nature ; and I am convinced it supplies the 

 clue to many otherwise puzzling facts in spider economy. However that 

 may be, the ascertained facts that the Pompilidas do not provision their 

 nests with ants, and that some of the species that persecute spiders have 

 special antipathy to these little insects, furnish convincing evidence of the 

 survival value of ant-mimicry to spiders ^. 



Let me not be misunderstood to suggest that it is only from the Pompilidse 

 that spiders are saved by this form of mimicry. I have no doubt that it 

 protects them also from the attacks of other enemies, notably from Ichneumon 

 flies of the genera Hemiteles, Polyspliincta, Acrodactylus and others, as well 

 as from various insectivorous mammals, birds and reptiles. But although 

 the annual mortality in the spider-world caused by these enemies, espe- 

 cially by the Ichneumonidse, is no doubt considerable, it probably sinks into 

 insignificance beside that brought about by the mason and digger wasps. 

 The view here advocated that ant-mimicry in spiders is protective was long- 

 ago insisted upon by Belt as well as by H. H. Smith f. But I am not aware 



* Since some species of the solitary wasps prey upon grasshoppers and other insects, this 

 suggested explanation of the value to spiders of their resemblance to ants applies also to the 

 ant-imitators found amongst Orthopterous, Homopterous, and other insects. 



t See Peckham, Occ. Papers Nat. Hist^ Soc. Wisconsin, 1889, p. 108, 

 LINN. JOUEN. — ZOOLOGY, VOL. XXX. 21 



