STOMODAEUM AND TROCTODAEUM. LABRUM. 293 



According to Graber, the law of the development of the body-segments from 

 before backward, which has been accepted on the whole for the Arthropoda 

 and was specially insisted upon by Balfour, does not apply to the Insecta. 

 In this group the germ-band is said to break up at first into macrosomites, 

 i.e., the slightly indicated swellings of the germ-band recognised by Ayers and 

 corresponding to the permanent regions of the body. The macrosomites are 

 said, by means of a secondary segmentation, to break up into microsomites 

 (the later body-segments). This peculiar type of segmentation, which deviates 

 from that of the other Arthropoda, is to be regarded as inherited from a hypo- 

 thetical racial form. AVe, however, are not able to accept this view. Apart 

 from the fact that in Hydrophilus (Heider), Chalicodoma (Carriers, No. 13), 

 Mantis (Viallanes, No. 84), and Xiphidium, one of the Locustidae (Wheeler, 

 No. 94), there is no sign of any breaking up into macrosomites preceding 

 definitive segmentation, it appears to us that the broadening of the germ- 

 baud at the part where later the thoracic region develops may be traced back 

 merely to an accumulation of plastic material, and that it should not therefore 

 be regarded as the expression of a true segmentation. If the lower layer were 

 also affected by this apparent breaking up into macrosomites, the case would 

 be different. Such a condition was actually stated by Graber to exist in 

 Stenobothrus (No. 26). From his more recent publication (No. 30), however, 

 it appears that the formation of macrosomites in the lower layer in Stenobothrus 

 is not cpiite distinct. We have therefore only the statements of Nusbaum 

 (No. 59) in connection with Meloe and, as no division into macrosomites is 

 found in Hydrophilus and Lina, the point seems to require reinvestigation. 



As a rule, the development of the body-segments in the germ- 

 hand of the Insecta takes place from before backward. This has 

 recently been observed, especially by Graber (No. 30), in various 

 forms (Stenobothrus, Hylotoma, Lina). In Lina, for instance, the 

 mandibular and the maxillary segments (Fig. 145 B, k'-k'") develop 

 first, and in the next stage the three thoracic segments and the 

 two anterior abdominal segments are added (Fig. 145 G), while 

 the other abdominal segments only develop later. In other cases, 

 the development of the segments seems to proceed more equally 

 along the whole length of the germ-band. Our knowledge is, 

 however, very incomplete on this point. An exception to the rule 

 is afforded by Hydrophilus, in which the development of the seg- 

 ments of a middle region is somewhat retarded, while the anterior 

 and posterior parts of the germ-band develop more rapidly. In 

 Pieris, according to Graber (No. 30), the thoracic segments precede 

 all the others in development. The maxillary segments soon follow, 

 and finally the abdominal segments form. 



B. Stomodaeum and Proctodaeum. Labrum. 



After the segmentation of the germ-band is completed, the next 

 ontogenetic changes to be remarked are the development of the 



