100 BEITISH LEPIDOPTERA. 



Some Liparid larvse, when alarmed, not only elevate their bristly 

 tussocks, but also, at the same time, expose a series of intensely black 

 intersegmental rings which, not at all conspicuous when the larva is 

 in repose, become so when it is disturbed. These help to make the 

 tussocks still more conspicuous. If such larvas are much disturbed 

 or attacked, they roll into a complete ring, the projecting tussocks 

 standing out as a bristly armature all round the body, and expose to 

 the full the black intersegmental membranes. Poulton's observations 

 on the larvte of Orgyia antiqua and Dasychira pudibunda, when attacked 

 by hungry lizards, led him to believe that their tussocks would pre- 

 serve the larva?, except in the case of a very hungry enemy. 



The urticating hairs of Lasiocampa guerciis form the short dorsal 

 fur of the larva, and are used to mix with the silk of the cocoon, 

 masses of them often lying loose in the outer web by which the cocoon 

 is attached to the twigs, etc., among which it is spun. Tbey also 

 s_tand out directly from the cocoon proper, and hence careless hand- 

 ling of the cocoon is almost sure to result in breaking off some of the 

 fine points in the skin. The urticating hairs of Macrothylacia rubi 

 appear to be very similar to, if not identical with, those of L. querciis. 



So far as our experience goes, urtication is purely the result of 

 the mechanical action of the hairs, but Swinton says that they are not 

 merely mechanical, and that the hairs are poisoned by a caustic liquid 

 issuing from the scarlet tubercles on the hinder segments of Porthesia 

 similis. This liquid has, however, been shown by Poulton to be odorous. 

 Bacot says that the urticating hairs of Porthesia similis and P. chrysor- 

 rltoea are much smaller than those of L. quercus, about one-third to 

 one- fourth of the length, and are much more complicated in struc- 

 ture, being spinose, and with the base divided into three prongs. In 

 Porthesia, they appear to be developed only on the tubercles, but in 

 the Lasiocampids, the dorsal and sub-dorsal areas of the larvae are 

 coated with them. He also agrees that their action is simply 

 mechanical. 



As it is not our intention to refer to the defensive properties of 

 hairs at greater length, we would call attention to the fact that all 

 spines or prickles serve to protect the organism from external attack. 

 They do not, of course, prevent many larva? being eaten, but they 

 prevent many animals preying upon them that might otherwise do so. 

 They are, therefore, to be considered as adaptive structures, and have 

 been developed from more simple, previously existent, structures, in 

 response to necessities, most probably arising from the attacks of 

 entomophagous animals. The changes that have occurred, therefore, 

 in the simple setiferous tubercles, or hairs, or in the minute body 

 hairs or pile, by means of which they have been respectively changed 

 into defensive spines, or fascicles, or into a dense hairy coat for pro- 

 tective purposes, form a very interesting and instructive line of study. 



Before concluding this chapter, it may be well to call attention to 

 an act of strict commensalism occurring among caterpillars. The 

 occurrence is recorded by Fritz Miiller, as taking place in South 

 America, between a large spiny caterpillar, which was almost inva- 

 riably found to be accompanied by a small hairy caterpillar, so small 

 as to rest securely in a transverse position across the back of its good- 

 natured host, well concealed among its spines, and it is stated that 

 the skin of the host was hardened at the place where the smaller larva 



