Class 3. Insecta. 249' 



the definite size is attained, ho-weyer, it apjDears suddenly to change- 

 its external form in a number of essential points^ and then after 

 moulting, it appears as a pupa. The pupa displays externally, a 

 very close approach to the imaginal form; the wings are fairly 

 prominent, and the compound eyes are also present, the legs and 

 antennae resemble those of the adult insect, and this is also the case 

 with the mouth-parts, but all the appendages are still somewhat^ 

 indefinite in outline, and not distinctly jointed ; like the incipient 

 limbs in the body of an embryo, they are not yet functional ; they 

 lie immovable upon the body, the general form of which is very like 

 that of the imago (relative development of thorax and abdomen, etc.) ;. 

 internally, however, the pupa at the moment of the last ecdysis, 

 is still in reality in the larval condition. The important changes, 

 which have occurred in external structure, have not, of course, taken 

 place so suddenly as they appear to have done ; they have all been 

 ready at the close of larval life ; the wings may, for instance, have 

 developed long before this as invaginations of the body-wall, and 

 when the larval skin is thrown off for the last time, they are 

 evaginated, and appear as outgrowths of the body; the legs have 

 already grown within their chitinous cases, and may be seen there, 

 folded up, towards the end of larval life. At this time, the insect is 

 inert, and remains as quiet as possible, for the modifications have 

 made its appendages, to a certain extent, functionless. The changes 

 are continued during the pupal stage ; the external form of the body 

 alters underneath the protective cuticle, and within, the larval organs 

 are gradually modified into those of the adult {see Fig. 210), so that 

 there is a considerable difference in the organs at the beginning and 

 end of pupation, although apparently the animal remains unaltered 

 during the whole time. Many pupee are quiescent ; true locomotion 

 occurs in only a few forms [e.g., Gnat-pupse, which are aquatic, and 

 must move about on the surface of the water to obtain air), and then 

 is brought about by movements of the abdomen, which also effect 

 respiration. When all the changes are at last concluded, the 

 chitinous pupa-case splits, and the imago emerges. When all 

 the appendages are unfolded, and the cuticle has hardened, the 

 development is in all essentials complete. The adult never 

 moults, it does not grow, or at least, not more than the chitinous 

 coat will permit. 



The changes wliich are undergone at the close of the larval period and during 

 pupation, are not hmited to a transformation of the parts already present, but in 

 addition, there is a general destraotion and dissolution of many larval organs ; in 

 some Insects, only a small portion of the larva aotuaUy forms the imago, whilst a 

 larger part imdergoes dissolution, and serves for the nourishment of the rest. 

 The amcBboid blood-corpuscles of the larva play an important part in this process, 

 in that they feed upon and digest the tissues as they die, passing on the nourish- 

 ment thus obtained to the actively growing organs of the animal. This process of 

 dissolution is carried to very great lengths in a large number of Diptera, whose 



