GENERAL CONSIDERATIONS. 395 



meta-thorax may be connected with the position of the centre of 

 equilibrium of the body. We agree with Brauer (No. 146) in 

 considering the wingless condition as a primary characteristic only 

 in the Apterygogenea, whereas in those wingless orders of Insects 

 (the Mallophaga, Siphonaptera, etc.) which are placed with the 

 Pterygogenea it must be regarded as secondarily acquired. 



The segmental arrangement of the tracheal stigmata should be 

 noted. It appears that originally a pair of stigmata occurred on 

 each of the three thoracic segments, as well as the eight following 

 abdominal segments, at least, the respiratory system of the Thysanura, 

 as investigated by Grassi and Haase, is favourable to such an 

 assumption. In most Insecta, however, the number of thoracic 

 stigmata is reduced. There does not appear to be a true pair of 

 stigmata in the head. We have already given the reasons (pp. 323 

 and 335) why neither the encloskeletal invaginations of the head nor 

 the salivary glands can be regarded as homodynamous with the 

 tracheal invaginations. It should, however, be mentioned, on the 

 other hand, that the presence of a pair of stigmata belonging to 

 the head has been maintained in Scolopendrella (Haase) and in 

 Sminthurus (Lubbock). 



We have still to mention the compound eyes (facet-eyes) as one of 

 the features which raise the Insecta to a higher level than the 

 Myriopoda. The most primitive form of eye in the Insecta is 

 evidently represented by the ocellus (Fig. 165, p. 332), the structure 

 of which, according to Grenacher (No. 151), may still, in a few 

 cases, be traced back direct to the simple cup-shaped eye, while, 

 in other cases, through the development of a vitreous body-layer 

 (lentigen layer), it becomes a bilaminar complicated eye (Fig. 164 B, 

 p. 331). We shall hardly err in deriving the Insectan ocellus direct 

 from the cup-shaped eyes of the Annelida (Kennel, No. 154). The 

 compound eye, on the contrary, appears to correspond to an accumu- 

 lation of ocelli, in which the number of ocelli has been increased 

 while the single ommatidia have sunk to a lower level of functional 

 capacity. We have already seen (p. 242) that, in the Myriopoda, an 

 almost complete series of transitions is to be found between the 

 aggregations of ocelli and the true facet-eyes. We shall therefore 

 be justified in assuming this derivation as highly probable for the 

 facet -eyes of the Insecta. For the relation of the facet -eye to 

 the ocelli of the same animal, cf. p. 333. Bearing in mind the 

 fact that Machilis already possesses facet -eyes, we must regard 

 the latter as a somewhat ancient acquisition among the ancestors 



