6 THE CLASS OF INSECTy. 



guishable from each other. Here we see the embryo divided 

 into u head-thorax and a tail. 



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

 life they are tiattened 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 

 1^ 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 

 Fig. 8. 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 liinder 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 (Mystucides) \a the esg, with part of the yolk 

 '(a;) not yet inc,lo.seil within the body-walls, n, antenna; ; between a and b the mandi- 

 bles; li, ni-.ixilla; c, laljiuin; d, the separate eye-spots (ocelli), which afterwards in- 

 crease greatly in nnniber and nnite to form the compound eye. The "neck" or 

 junction of the liea<l with tlie 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 flhiments in the Cockroach and May-fly, etc. The rings of the body are 

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

 Here, as in the other two flgures, though not so distinctly seen, the antenna^ 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 directed forwards, as 

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

 "downwards, since they are to support the insect while walking. Itappears that the 

 two ends of the V)ody are perfected before the middle, and the under side before the 

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

 above. Thus all articulates difl'er from all vertebrates in having the yolk-mass 

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

 bryo. — From Zaddach. 



