132 NATURAL HISTORY OF ARTHROPODS. 



The abdomen, which encloses most of the vegetative and all of the reproductive 

 organs, is composed of ten segments which are but rarely provided with appendages 

 in the adult condition. An exception to this may be noted in the fact that the stints 

 and ovipositors of many forms are in reality composed of the modified appendages of 

 the last three segments of the abdomen. 



Among the most striking features of the Hexapoda are the wonderful meta- 

 morphoses undergone by many members of the group in passing from the larval 

 to the adult condition. A few forms are ovoviviparous, that is bring forth living 

 young, but the great majority lay eggs which in the course of time hatch out the 

 young. The eggs of many forms are very interesting from the peculiar and beautiful 

 ways in which their surface is ornamented. The eggs undergo a partial segmenta- 

 tion, the central portion not dividing, and soon from the cells thus produced an 

 envelope arises which from being formed in the same way as one of the foetal envelopes 

 in the mammalian embryo has received the name amnion. This finally envelops the 

 whole of the egg ; soon a portion of the egg proper becomes differentiated, forming an 

 embryonic disc which soon becomes divided into the future segments of the body. 

 On each of these primitive segments there frequently appears a pair of small swellings, 

 the rudiments of the future limbs. Frequently, however, some of the segments show 

 at no time any trace of appendages. With the progress of growth the embryo acquires 

 what is known as a larval condition, and then the egg hatches and the young begins a 

 free life. 



The term larva is applied to very varying stages of development as far as organs 

 and perfection of parts are concerned, but the word may be defined in general terms 

 as applying to the young insect just after hatching. In the grasshoppers, for instance, 

 the larva closely resembles the adult, the most prominent difference being that the 

 wings are extremely rudimentary. In the Diptera, on the other hand, the body is 

 long and slender and without limbs, and, in some forms, even a distinct head is lack- 

 ing. Between these two extremes almost all gradations can be found. The larvae of 

 Diptera (flies) are known as maggots, of the Lepidoptera (butterflies and moths) as 

 caterpillars, and those of the Coleoptera (beetles) as grubs. The early stages of the 

 other orders have received no popular names. 



Between the larval and adult conditions the features of growth are as different as 

 the characters of the larvae, but the phenomena exhibited may be roughly divided into 

 three groups. In the first (ametabolic) the insects undergo no metamorphosis, and 

 the adult, which never has wings, differs from the larva in little else except size. The 

 second (hemimetabolic) type passes through what is known as an incomplete metamor- 

 phosis. The larva increases in size by numerous moults and passes into a pupal con- 

 dition, in which the wings are rudimentary and the general adult form is apparent. 

 This pupa, however, is markedly different from that of the next group, in that they are 

 active instead of quiescent. This type of development is exemplified in the grass- 

 hoppers, dragon-flies, bugs, etc. 



The last, the holometabolic or complete metamorphosis, occurs among the ^bees, 

 butterflies, flies, beetles, etc. We may take for an example of this type one of our 

 American silk-worms. The larva upon hatching eats voraciously, the consequent 

 growth being permitted by numerous moults, until at last the young insect is ready to 

 enter the pupal stage. The larva has mouth-parts adapted for biting, threeji* pairs of 

 thoracic legs, while five of the abdominal segments bear locomotive appendages known 

 as pro-legs. During the later stages the wings of the perfect insect begin *''cir devel- 



