NO. 1687. THE THORAX Of INHEVTS—iSNODGRAm. 515 



tioii of tho poison claws and the labial palpi of the chilopods would 

 form an oi-yan much more nearly resembling the insect maxillir than 

 would a union between the poison claws and the first maxilhv of 

 chilopods. 



All i-easoning of this fascinating sort, however, simply shows the 

 limitless extent to which morphological theorizing can be carried on 

 anatomical grounds. 



Verhooff's theory of head segmentation has' been severely criticised 

 by Ileymons (100.")) on the ground that the facts of embrj^ology 

 utterly refute it. and that it does not conform with the segmentation 

 of the nervous system. 



The simplest embryological view holds that there are four segments 

 in the head — a preoral, a mandibular, a maxillary, and a labial seg- 

 ment. This is advocated by Lowne (1892), who regards the three 

 embryonic divisions of the preoral region as secondary. Bengtsson 

 (1897, 1905) adopts this view concerning the preoral region, but he 

 finds four segments in the postoral part of the head. Almost all 

 students of the embryology of the insect head, however, regard the 

 three preoral divisions as true metameres. Hence, embryologists are 

 divided in opinion mainly between six and seven head segments. 

 The principal advocates of six segments are Zaddach (1854), Hux- 

 ley (1878), Yiallenes (1887), meeler (1893), Heymons (1895), 

 Packard (1898), Riley (1904), and Holmgren (1904, 1907). The 

 advocates of seven head segments are Folsom (1899, 1900) and 

 Comstock and Kochi (1902). But these authors are supported also 

 by Bengtsson (1897, 1905) and by Borner (1904) in so far as they 

 find four postoral segments, though they recognize only one preoral 

 segment. 



The seven-segment theor_y is based mainly on Folsom's (1900) ob- 

 servation that seven pairs of ganglia appear in the head soon after 

 involution, and that in Anurida maritima a pair of appendages or 

 " superlingua^ *' appear back of the mandibles, corresponding with 

 the fourth pair of ganglia. These appendages fuse in most insects 

 with the lingua of the embryo to form the hypopharynx of the adult, 

 Ijut in many lower forms they remain as the lateral lobes of the 

 hypopharynx or " endolabium " and have been misleadingly called 

 the " paraglossa?.'' Borner (1904) finds that the hypopharynx of 

 nearly all insects having incomplete metamorphosis is a compound 

 structure formed of the median "• glossa'' and the hiteral paired ele- 

 ments, which he calls the " maxillulse." (The reader must remember 

 that the terms " glossa " and " paraglossse " have been inconsiderately 

 applied by some recent entomologists to the parts of the hy])0]:)harynx 

 or " endolabium," while they properly belong to the outer or true 

 labium.) Borner thus recognizes four postoral segments. Hansen 

 (1893) suggested that the ^' paraglossse " (superlingua3, maxillulas) of 



