94 BRITISH LEPIDOPTERA. 



ft 

 that the formidable spines of the grown-up caterpillar save it not 

 infrequently from being swallowed by birds, though the younger larvae 

 appear to rely rather on their movement for scaring their enemies. 



Closely allied to the mutant structures just described, so far as their 

 protective value is concerned, are the eversible glands found in various 

 caterpillars. One of the most general of these is the chin-gland. This 

 is a swollen vesicle situated on the ventral side of the prothorax, just in 

 advance of the first pair of legs. It is very generally developed in 

 Noctuid, Notodontid and butterfly larvae, and appears to be the remnant 

 of an everted gland, and is probably homologous with the active 

 weapon of offence found in the same position in the larva of Centra 

 vinula which has the power to eject from it, with considerable 

 force, a quantity of formic acid. The chin-gland is found in almost 

 all butterfly larvae which do not possess osmateria. It is, usually, 

 bladder-like and vesicular, is extensible, and can be protruded and with- 

 drawn, and has been by some authors considered to have some con- 

 nection with the osmateria of Papilionid larvae. That the osmateria 

 and chin-gland at one time had a somewhat similar function, is very 

 probable, though it is difficult to explain why the latter should have 

 degenerated so completely in some species. When withdrawn, the 

 chin-gland presents only a transverse slit, similar to that left when 

 the osmateria of the Papilionid larvae are withdrawn. The chin-gland 

 has been long known to entomologists, certainly as far back as Bonnet's 

 time, a century and a half ago. 



It may be well to examine, somewhat in detail, the chin-gland of 

 Centra vinula. This, like that of the allied species, C. furcula, consists 

 of four lateral processes, two on each side, and a central sac, in which 

 the irritant secretion is stored. The sacs can be readily pressed out in an 

 almost full-fed larva. Poulton considers that the gland could originally 

 have been everted voluntarily, but that the power has been lost since it 

 has acquired the ability of secreting an irritant fluid. The fluid 

 ejected by the larva of C. vinula has been chemically analysed, and has 

 been found to consist of formic acid. It affects litmus paper strongly, 

 and causes effervescence when it comes in contact with bicarbonate of 

 soda. Crystals of formate of lead, obtained by collecting the secretion 

 of the larva on 283 occasions, were exhibited at a meeting of the Ento- 

 mological Society of London. The secretion had been mixed with 

 distilled water, in which oxide of lead was suspended. The latter 

 dissolved, and the acid of the secretion being in excess, the normal 

 formate was produced (Trans. Ent. Soc. Lond., 1887, p. xxxvi). It 

 has also been recorded that the larva of Stauropus fagi ejects an acid 

 fluid in a somewhat similar manner to that ejected by C. vinula, and 

 that on one occasion it caused considerable pain, owing to the liquid 

 being squirted in the eye (Entom. Piecord, iv., p. 32). 



The osmateria of the Papilionid and Parnassid larvae are found on 

 the dorsal surface of the prothorax. Each consists of a bifurcate sac, 

 which, ordinarily concealed within a cavity of the prothorax, and 

 closed normally by a transverse slit, is suddenly thrust out when the 

 larva is disturbed. As observed in the larva of Papilio machaon, it 

 consists of two fleshy protuberances branching from a common stalk, 

 is of an orange colour, eversible, gives out a strongly-scented odour 

 (and a drop of clear liquid, according to Poulton) every time it is 

 protruded. It is, in other species, usually brightly coloured, and 



