548 TEXT-BOOK OF ENTOMOLOGY 



antennse), and afterwards by Tichomiroff in Lepidoptera. Heider, in his work 

 on Hydrophilus, describes it as the "lateral mouth-lips," while, more recently, 

 Nusbaum has observed it in Meloe. This under lip structure may be regarded 

 as analogous to the paragnaths of Crustacea, although to attempt to homologize 

 it with these seems useless. (Korschelt and Heider.) 



Completion of the head. Sufficient attention has not been paid to 

 this subject by embryologists. The head is at first, dorsally, mostly 

 composed of the head-lobes, or antennal segment only, and the 

 dorsal or tergal portion of the oral appendages develop at a later 

 period. We have observed in the embryo of dragon-flies (yEschna) 

 that the tergites of the mandibles and first maxillae are simul- 

 taneously fused with the head-lobes, while the much larger tergal 

 region of the 2d maxillae remains for some time separate from the 

 anterior part of the head, and is continuous with the thoracic seg- 

 ments, and it is only just before hatching that this segment becomes 

 fused with the rest of the head (Fig. 36). In a sense, the 2d 

 maxillary segment when it is free from the head reminds us of the 

 foot- jaw, or 5th segment of chilopod myriopods (see also p. 53). 



g. The appendages 



As we have seen, nearly or quite simultaneously all the limbs 

 as a rule bud out from each side of the median line of the primitive 

 band. They arise as saccular evaginations or outgrowths of the 

 ectoderm, directed a little backwards. They are at first filled with 

 mesoderm cells, and in the Orthoptera diverticula of the coelom- 

 sac are taken up into the rudimental limbs, as in Peripatus and 

 Myriopoda. (Graber, Cholodkowsky.) As the antennae, mouth- 

 parts, legs, and abdominal appendages are all alike at first, their 

 strict homology with one another is thus demonstrated. In insects 

 never more than a single pair of limbs is known to arise from one 

 segment. 



The cephalic appendages. - - The antennae evidently arise from the 

 hinder edge of the procephalic lobes (Fig. 527, an). As in Limulus, 

 the first pair of appendages are at first post-oral (Fig. 528, at), 

 afterwards moving forward owing to changes in the relative propor- 

 tions of the parts of the head, and they are in all respects, in their 

 development and position in relation to the segment from Avhich 

 they arise, homologous with the appendages succeeding them. 



The occurrence of rudiments of a pair of prrantennal appendages in Chalico- 

 doma which is claimed by Carriere, needs confirmation, as other embryologists 

 have not observed them. 



