334 INSECTA. 



with the pupae of the holometabolic Insecta (Diptera, "Weismann, 

 No. 129; Lepidoptera and Hymenoptera, Carriere, No. 147). In 

 many cases, the first rudiment of the facet-eyes, as we saw above, 

 is a paired lateral ectoderm -thickening, while in others, only a 

 crowding together of the individual ectoderm -cells can be seen. 

 The separation of the single ommatidia (single eyes) here takes 

 place exclusively through histological differentiation (Fig. 166). At 

 an early stage, as described in connection with Mysis (Vol. ii., 

 p. 169), the single ommateal pillars and the undifferentiated tissue 

 between them, which in Vespa form very massive intermediate 

 pillars, can be distinguished. In the region of the ommateal 

 pillars the cells become arranged to form two layers, the outer 

 yielding the cells of the crystalline cone (2) and the principal 

 pigment-cells (<•?), while the inner yields the retinulae (4), which 

 are connected with the nerve-fibres (-5). The cells of the crystalline 

 cone secrete outwardly the cuticular corneal lens, while the crystal- 

 line cone develops within them in eyes of the eucone type. The 

 cells of the intermediate pillars (i) give rise to the so-called 

 accessory pigment-cells. In the course of further development the 

 optic rudiment thickens considerabl}", the single ommatidia thus 

 becoming taller and narrower, and also shifting closer together. The 

 retinula-cells especially gain greatly in height. Pigment becomes 

 deposited both in the retinula-cells and in the various pigment-cells 

 which cover the outer side of each ommatidium. The development 

 of the most essential features of the ommatidia seems to be thus 

 completed (Carriere). 



E. The Tracheal System. 



The tracheae arise as paired, segmentally arranged ectodermal 

 invaginations lying laterally to the limb-rudiments (Fig. 146 A, st, 

 p. 295; Fig. 147 A st, p. 297; and Fig. 158 E, tr, p. 321). The 

 tracheal invaginations are usually found developing at a somewhat 

 early^stage soon after the appearance of the limb-rudiments. It, 

 however, appears from one of Grassi's observations (No. 33) that, 

 in Japyx, the tracheal system does not develop until a late embryonic 

 stage. This would have to be regarded as the more primitive con- 

 dition, recalling the Myriopoda (pp. 243 and 254). For since the 

 tracheal system, phylogenetically considered, represents one of the 

 latest acquisitions of the racial form, its early development in most 

 Insects must be considered as being secondarily shifted back to early 

 stages on account of its importance. 



