NUMBER OF SEGMENTS IN THE HEAD 



53 



of centipedes (Chilopods), is shown by our observations on the em- 

 bryology of yEschna (Fig. 36). 



The niandibular segment appears to form a large part of the post- 

 antennal region of the epicranium on account of the great niandibu- 

 lar muscle which arises from so large an area of 

 the anterior region of the head (Fig. 37). 



Judging from the embryo of Nematus (Fig. 37), 

 the first maxillary segment is tergally aborted, there s 

 being 110 tergo-pleural portion left. 1 



The second maxillary segment tergally appears 

 to be represented by the occipital region of the 

 head. 



All the gular region, including the submentum 

 and mentum, probably represents the base of the 

 labium or second maxillae. 2 The so-called "occi- 

 put" forms the base of the 

 head of Corydalus, a neurop- 

 terous insect, which, however, 

 is more distinct in the larva. 



FIG. 36. J5schna 



Iii most other adult insects the near 'y rea(J y to hatch = 



4, labium, between T 



occiput is either obsolete or and the occipital ter- 



1 gite ; 5-7, legs. 



fused with the hinder part of 

 the epicranium. We have traced the history of 

 this piece (sclerite) in the embryo of yEschna, 

 a dragon-fly, and have found that it represents 

 the tergal portion of the sixth or labial seg- 

 ment. In our memoir on the development of 

 this dragon-fly, PL 2, Fig. 9, the head of the 

 embryo is seen to be divided into two regions, 

 the anterior, formed of the antennal, niandibular, and first maxillary 

 segments, and the posterior, formed of the sixth or labial segment. 



FIG. 37. Head of em- 

 bryo Nematus, showing 

 the labial segment : occ., 

 forming the occiput; cl, 

 clypeus ; Ib, labrum ; md, 

 mandible ; mdm, muscle of 

 same ; nix, maxilla ; m&', 

 second maxilla (labium) ; 

 oe, oesophagus. 



1 While these pages are still in type, we may add, in confirmation of this view, that 

 Uzel states, from his researches on the embryology of Campodea, that the maxillary 

 tergites of the embryo only slightly share in building up the tergal region (occiput) 

 of the head, but that they form the geuae of the maxillary segments. (Zool. Anzeiger, 

 July 5, 1897, p. 235.) 



2 Miall and Denny in their Avork on the cockroach, in describing the labium, remark : 

 " The upper edge is applied to the occipital frame, but is neither continuous with that 

 structure nor articulated thereto. By stripping off the labium upwards it may be 

 seen that it is really continuous with the chitinous integument of the neck" (p. 95). 

 This is, we think, a mistaken view, as proved by the embryology of the Odonata and 

 of Nematus. Our statements on this subject were first published in part in 1871, and 

 more fully in the third Report, U. S. Ent. Commission, 1883, pp. 284, 285. We also 

 stated that all the gular region of the head probably represents the base of the primi- 

 tive second maxillae. 



