6 THE CLASS OF INSECTS. 



guishable from each other. Here we see the embryo divided 

 into a head-thorax and a tail. 



It is the same with Insects. Within the egg at the dawn of 

 life they are flattened oblong bodies curved upon the yelk- 

 mass. Before hatching they become more cylindrical, the 

 limbs bud out on the sides of the rings, the head is clearly 

 demarked, and the young caterpillar soon steps forth from the 

 egg-shell ready armed and equipped for its riotous life. 



As will be seen in Fig. 8, the legs, jaws, and antennae are 

 first started as buds from the side of the rings, being simply 



elongations of the body-wall, 

 which bud out, become larger, 

 and finally jointed, until the 

 | a? buds arising from the thorax or 

 abdomen become legs, those 

 from the base of the head be- 

 come jaws, while the antennae 

 and palpi sprout out from the 

 front rings of the head. Thus 

 while the bodies of all articulates 

 are built up from a common em- 

 bryonic form, their appendages, which are so diverse, when we 

 compare a Lobster's claw with an Insect's antenna, or a Spider's 

 spinneret with the hinder limbs of a Centipede, are yet but 

 modifications of a common form, adapted for the different uses 

 to which they are put by these animals. 



FIG. 8. A Caddis, or Case-fly (Mystatides) in the egg, with part of the yolk 

 (a:) not yet inclosed within the body-walls, a, antennae ; between a and b the mandi- 

 bles; ft, maxilla; c, labium; rf, the separate eye-spots (ocelli), which afterwards in- 

 crease gi'eatly in number and unite to form the compound eye. The "neck" or 

 junction of the head with the thorax is seen at the front part of the yolk-mass; e, 

 the three pairs of legs, which are folded once on themselves ;/, the pair of anal legs 

 attached to the tenth ring of the abdomen, as seen in caterpillars, which form long 

 antenna-like filaments in the Cockroach and May-fly, etc. The rings of the body are 

 but partially formed; they are cylindrical, giving the body a worm-like form. 

 Here, as in the other two figures, though not so distinctly seen, the antennae, jaws, 

 and last pair of abdominal legs are modifications of but a single form, and grow 

 out from the side of the body. The head-appendages are dii'ected forwards, as 

 they are to be adapted for sensory and feeding purposes ; the legs are directed 

 downwards, since they are to support the insect while walking. It appears that the 

 two ends of the body are perfected before the middle, and the under side before the 

 upper, as we see the yolk-mass is not yet inclosed and the rings not yet formed 

 above. Thus all articulates differ from all vertebrates in having the yolk-mass 

 situated on the back, instead of on the belly, as in the chick, dog, or human em- 

 bryo. From Zaddach. 



