GENERAL CONSIDERATIONS. 393 



sequent anal segment (telson). Greater difficulty arises in reckoning 

 the number of segments which have been drawn into the formation 

 of the head. Three maxillary segments (a mandibular and a first 

 and second maxillary segment) here combine with an anterior primary 

 cephalic section. The segmentation of the brain leads us to suppose 

 that the latter is composed of three segments (p. 325), while between 

 this section and the mandibular segment a vestigial so-called 

 pre-maxillary segment seems to be intercalated. In reckoning the 

 segments here, however, we are on somewhat hypothetical ground. 

 It may be mentioned that the antennae belong to the second brain- 

 segment, and, by their originally post-oral position, as well as by 

 their relation to the coelomic sacs belonging to that segment (in 

 Orthoptera, p. 295), in all respects resemble true trunk-limbs. This 

 is in entire agreement with what has been learnt of these limbs in 

 connection with Peripatus and the Myriopoda. 



One of the most interesting questions in the phylogeny of the 

 Insecta is that of the rise of the wings. The rudiments of the wings 

 appear on the meso- and meta-thorax as dorsal mtegumental out- 

 growths, the inner cavities of which receive later the tracheal 

 ramifications. It is an interesting fact that similar lateral fold-like 

 widenings of the dorsal plates, which recall the first rudiments of 

 wings, also occur on the pro-thorax (Machilis and Blatta). These 

 are most clearly visible on the larvae of Calotermes (Fig. 194, 

 F. Muller, Xo. 158), in the youngest stages of which outgrowths 

 of the pro-thorax and meso-thorax are first evident, these being 

 originally devoid of tracheae. While the anterior pair of these 

 outgrowths degenerates, the posterior pair is supplied with tracheae, 

 and is thus transformed into the rudiment of the fore-wing, the 

 rudiment of the hind-wing appearing simultaneously on the meta- 

 thorax. The great similarity in position and structure between the 

 wing-rudiments and the leaf-shaped tracheal gills, as found on the 

 abdominal segments of the Ephemerid larvae (Fig. 177, /', p. 358), 

 has led to many attempts to consider them as homodynamous 

 structures. This view, which was adopted by Gegenbauek and 

 Lubbock (Xo. 156), has recently also received the support of 

 Reetenbacher (Xo. 165). F. Muller, who also supports the 

 above, is inclined to hold that the original function of the wings 

 was respiratory. This view, which seems well supported by the 

 structure of the wing-rudiments, within which are found blood- 

 spaces and tracheal ramifications, involves the assumption that the 

 winged Insects are derived from an aquatic form. The phyletic series 



