52 



THE BRITISH NATURALIST. 



[March 



but their compressed and coalesced representatives, the four pairs of 

 pulmonary sacs, perform the respiratory function. In spiders, tracheae, 

 as such, persist, but there are, in addition, one or two pairs of respiratory 

 sacs. 



Passing to the Thysanura, which are often described as primitive 

 insects, we find a class which in some measure justifies the use of that 

 expression, though it cannot be regarded as representing the ancestral 

 type of the true insects. Thysanurans possess antennae, mouth parts 

 comparable to those of true insects, three pairs of legs, and a tracheal 

 breathing system ; but there is not the slightest trace of wings. This 

 proves, or at least renders it highly probable, that the Thysanuran, like 

 the Arachnidan, is the descendant of a terrestrial form; that there has not 

 been any aquatic form in its ancestry since the emergence from the worm 

 type. Some of the species have legs on the abdominal somites, suggest- 

 ing relationship to the Myriapods. These abdominal legs occur in the 

 adult, and not merely in the embryonic form as is the case with spiders 

 and scorpions. As showing the intimate relationship of the air-breathing 

 Arthropods, we may point out, that if the two legs which in the Thysanura,. 

 as in true insects, are modified to form the labium, that is the lower 

 portion of the mouth, had retained their primitive form of ambulatory 

 instead of biting organs, then the Thysanurans instead of being described 

 as primitive insects would have been transferred to the other class, and 

 called primitive Arachnidans. 



We have so far been treating of the apterous Arthropods, we come 

 now to the winged members of this group, which constitute the class 

 Insecta, which contains by far the greatest number of species and 

 individuals of any class of animals. Of course there are some apterous 

 insects, but in all there is good reason for saying that the wingless are 

 descended from winged forms. This fact alone is decisive, and places it 

 beyond doubt that the whole class Insecta are descended from aquatic 

 ancestors ; aquatic ancestors not in the sense in which all animals are so 

 descended, but in the sense that some descendants of the primitive 

 peripatus-like Arthropods which, as we have seen, were terrestrial forms 

 became adapted for aquatic life. 



This change of environment necessitated and was accompanied by 

 changes in the structure of the respiratory system. The tracheal system 

 did not cease to be strictly a tracheal system," but the mouths of the 

 tracheae became closed so as to exclude the water, and some of them, 

 perhaps all, were surrounded by leaf-like outgrowths similar both in 

 structure and function to the gill-leaflets of many modern ephemerid 

 larvae. It has been shown how from an ancestor which had 

 gill-leaflets down each side of its body there arose forms in 

 which the thoracic leaflets were larger, and since all these leaflets possess 1 

 some power of movement the transition from the gill-leaflet, a breathing 



