410 INSECTA. 



gosus (one of the Termites) that peculiar and similar dorsal appendages are 

 present on the two anterior of the thoracic segments. They are without 

 tracheae. The anterior atrophies, and the posterior acquires tracheae and gives 

 rise to the first pair of wings. The second pair of wings is formed from 

 small processes on the third thoracic segment like those on the other two. 

 Fritz Miiller concludes from these facts that the wings of Insects are 

 developed from dorsal processes of the body, not equivalent to the ventral 

 appendages. What the primitive function of these appendages was is not 

 clear. Fritz Miiller suggests that they may have been employed as respira- 

 tory organs in the passage from an aqueous to a terrestrial existence, when 

 the Termite ancestors lived in moist habitations a function for which pro- 

 cesses supplied with blood-channels would be well adapted. The undoubted 

 affinity of Insects to Myriapods, coupled with the discovery by Moseley of a 

 tracheal system in Peripatus, is however nearly fatal to the view that Insects 

 can have sprung directly from aquatic ancestors not provided with tracheae. 

 But although this suggestion of Fritz Miiller cannot be accepted, it is still 

 possible that the processes discovered by him may have been the earliest 

 rudiments of wings, which were employed first as organs of propulsion by a 

 water-inhabiting Insect ancestor which had not yet acquired the power of 

 flying. 



The nervous system. The nervous system arises entirely 

 from the epiblast ; but the development of the pra>oral and 

 post-oral sections may be best considered separately. 



The post-oral section, or ventral cord of the adult, arises as 

 two longitudinal thickenings of the epiblast, one on each side of 

 the median line (fig. 189 B, vn), which are subsequently split off 

 from the superficial skin and give rise to the two lateral strands 

 of the ventral cord. At a later period they undergo a differenti- 

 ation into ganglia and connecting cords. 



Between these two embryonic nerve cords there is at first a shallow 

 furrow, which soon becomes a deep groove (fig. 189 C). At this stage the 

 differentiation of the lateral elements into ganglia and commissures takes 

 place, and, according to Hatschek (No. 414), the median groove becomes in 

 the region of the ganglia converted into a canal, the walls of which soon fuse 

 with those of the ganglionic enlargements of the lateral cords, and connect 

 them across the middle line. Between the ganglia on the other hand the 

 median groove undergoes atrophy, becoming first a solid cord interposed 

 between the lateral strands of the nervous system, and finally disappearing 

 without giving rise to any part of the nervous system. It is probable that 

 Hatschek is entirely mistaken about the entrance of a median element into 

 the ventral cord, and that the appearances he has described are due to 

 shrinkage. In Spiders the absence of a median element can be shewn with 

 great certainty, and, as already stated, this element is not present in 



