292 INSECTA. 



region carries the oral aperture, and is characterised by its great 

 lateral extensions, the cephalic lobes (Figs. 134, k and 145, M), 

 while the posterior terminal section, the so-called anal segment or 

 telson, carries the anal aperture (Fig. 146 A, a). Between these 

 two regions lies the segmented primary trunk-region, which, in the 

 Insecta, seems without exception to consist of sixteen segments. 

 The three anterior of these segments represent the mandibular and 

 the two maxillary segments, which are later drawn into the forma- 

 tion of the head (Fig. 146, mcl, mx v mx 2 ), while the three following 

 develop into the permanent thoracic segments (j? v p 2 , ^ 3 ), so that 

 ten segments (besides the telson) must be reckoned as belonging 

 to the posterior or abdominal region of the body. 



Ten abdominal segments together with a telson seem typical throughout the 

 group of the Insecta. This number has been observed recently in the germ- 

 band of Hydrophilus by Heider, and in various forms (Lina, Stenobothrus, 

 various Lepidoptera, and Hylotoma) by Graber (No. 30). Wheeler (No. 95), 

 Cholodkowsky (No. 19), and Careiere (No. 13) have all made similar 

 observations. In the later stages of embryonic development, this number is- 

 apparently in a few forms decreased to nine, the tenth abdominal segment 

 fusing with the telson. This appears to be the case in Hydrophilus and Lina ; 

 in the Lepidoptera, according to Graber (No. 30), a fusion cf the ninth and 

 tenth abdominal segments takes place, the telson remaining independent. 



With regard to the primary cephalic region, it should be mentioned that,, 

 taking into account the segmentation of the brain recently observed by Patten 

 (No. 67) and confirmed by several other authors, it has to be assumed that this- 

 region is composed of several (three) fused segments (cf. pp. 325-328 on the 

 development of the brain). 



Another point to be noted is that, according to the statements of various 

 authors, among whom Wheeler and Carrieke deserve special mention (the 

 former in connection with Boryphora, No. 95, and the latter with Chalicodoma, 

 No. 13), a slightly developed and transitory segment, the so-called pre-maxillary 

 segment, is intercalated between the primary cephalic region and the first body- 

 segment jtroper (which represents the mandibular segment). According to- 

 Carrieke, this structure represents a vestigial pair of limbs and a corre- 

 sponding pair of ventral ganglia. The latter is said to be concerned in the- 

 formation of the circum-oesophageal commissure. 



The cephalic lobes usually appear very early (Fig. 145, M). Even when 

 the germ-band is still altogether devoid of segmentation, the primary cephalic 

 region is ahead}' characterised by the extensions of the cephalic lobes. A 

 slight broadening can also often be observed in that part of the still unseg- 

 mented germ-hand which corresponds to the later thoracic segments (Fig. 145 

 A and B, th). Indeed, Ayers (No. 1) was able to distinguish in the still 

 unsegmented germ-band of Oecanthus a primary cephalic region, a maxillary, 

 a thoracic, and an abdominal region, these later regions of the body being 

 indicated by variations in the bulk and breadth of the germ-band. It is on 

 these first rudiments of the body-regions, which are only recognisable as wavy 

 swellings of the lateral contour of the germ-band, that Graber (Nos. 26 and 

 30) founds his view of the primary segmentation of the Insectan germ-band^ 



