markings and attitudes of lepidoptei'ous larva. 321 



while the fluid was aiopHed to the ventral surface as well 

 as to all other parts. At the same time it is very likely 

 that the moistened part ma} , while wet, appear different 

 in colour from the dry part. The change of skin at 

 pupation does not essentially differ from an ordinary 

 ecdysis, and it is therefore difficult to understand why 

 the skin should receive such elaborate preparation for 

 the former event only. It is to be hoped that further 

 observation may decide upon the frequency of the occur- 

 rence and lead to some suggestion as to its use. 



16. An anatomical reason for the special protection 

 OF LARv^. — Larvae differ from most other organisms in 

 their liability to death from slight injuries. The reason 

 for this is to be chiefly found in the anatomical con- 

 struction of a larva, which may be described as a soft- 

 w^alled cylindrical tube which owes its firmness, and, 

 indeed, the maintenance of its shape, to the fact that it 

 contains fluid under considerable pressure. The pressure 

 is exerted by the muscular parieties of the body. The 

 advantage of this construction is as obvious as its 

 danger : the larva possesses a motive force which can be 

 applied to any movable part of the surface through the 

 medium of the fluid. Indeed, it does not seem possible 

 that the emission of a process of the body-wall could be 

 effected with any great power under any other system of 

 construction, at any rate, in soft-bodied animals. And 

 it is necessary that larvae should thrust out various 

 projections with great force. Thus the claspers must 

 retain the larva (often of considerable weight) upon the 

 food-plant during high winds ; and the force with which 

 they hold is seen in the fact that larvae may be often 

 injured by roughly and rapidly detaching them. Again, 

 many larvae possess flagella or shorter projections, which 

 must be swiftly emitted for the purpose of driving away 

 ichneumon flies, &c. Then there are fluid secretions, 

 which must be ejected with considerable force, and glands 

 producing a disagreeable odour, which are bodily everted 

 at a moment's notice (larvae of certain phytophagous 

 Hymenoptera). This motive force is also known in 

 very different organisms: the eye-bearing "tentacles" 

 of the snail are thrust out by such means. The 

 retraction of all processes which are emitted in this way 

 must be by invagination, and this is most readily 

 performed by means of an axial muscle attached to the 



