THE INSECT HEAD SNODGKASS 



453 



Pre. 



existed, we may call the phylogenetic equivalent of the embryonic 

 head the fvotocefhalon. The protocephalon is preserved as the en- 

 tire adult head in many Crustacea, as in the phyllopods (fig, 2 A, 

 Pre) and in the shrimps, crabs, and lobsters (fig. 16 A, Pre). In 

 insects, chilopods, diplopods, and some crustaceans it forms only the 

 procephalic part of the definitive head. 



Looking again at an insect embryo in a late protocephalic stage 

 of its head development (fig. 5 D), we observe that the body seg- 

 ments immediately following the cephalic lobes have each a pair 

 of well-developed appendage rudiments (iJ/c?, IMx^ 2Mx). These 

 appendages do not differ at first from the leg rudiments (/>), but 

 they do not keep up in growth with 

 the legs, and in some insects the region 

 of the legs, or thorax (fig. 6, Th)^ is 

 soon differentiated from a region of 

 three segments {Gn) between the tho- 

 rax and the protocephalon {Pre). The 

 appendages of these segments {Md^ 

 iMx, 2Mx) become the mandibles and 

 the two pairs of maxillae of the adult 

 insect. The part of the embryonic 

 body bearing them is termed, there- 

 fore, the gnathal region. Before the 

 insect embryo hatches, the gnathal 

 segments are united with the cephalic 

 lobes, and all the parts thus brought 

 together are consolidated in the defin- 

 itive head. Tlie intersegmental lines, 

 with the possible exception of the line 

 between the two maxillary segments, 

 are completely obliterated, while the 

 surface of the mature head capsule 

 becomes secondarily subdivided by the lines of cuticular inflections 

 forming internal ridges which strengthen its walls. The gnathal 

 appendages constitute the principal external feeding organs of the 

 insect. Being crowded forward on the ventral side of the head, 

 the first pair, or mandibles, come to lie at the sides of the mouth, 

 and are transformed into jawlike organs having a biting and 

 chewing function. 



There is no doubt, from the embryological evidence, that the insect 

 head has been formed from the union of two regions of the primi- 

 tive trunk, the first region, bearing the eyes, the labrmn, the mouth, 

 and the antennae, being the proeephalon, the second, bearing the 

 gnathopods and the hypopharynx, the gnathocephalon. The idea 



Figure 0. — An insect embrj'o show- 

 ing distinct differentiation of tlie 

 body into four parts, protocepha- 

 lon (Pre), gnathal region (On), 

 thorax (Th), and abdomen (Ah). 

 Embryo of Ranatra fusca (from 

 Hussey, 1026) 



