ADAPTATIONS TO HAUNTS AND SEASONS 267 



tatus — are not known to live anywhere except in cave 

 galleries, others like Pseudosinella caver narum and Arrhopaltes 

 caecus are found also in less profound dark dwelling-places, 

 such as ants' nests, moles' nests, and quarry- tunnels or 

 under large, deeply imbedded boulders. 



These distributional facts about springtails have been 

 mentioned in order to emphasise the exceedingly wide range 

 of these frail, lowly insects, whether one considers the order 

 as a whole or many of its component species. If we pass 

 on to inquire the reason of this remarkable adaptability to 

 surroundings often apparently so diverse, the answer seems 

 to be furnished by the small size and comparative simplicity 

 of form and function in these insects, which enable them to 

 survive, increase, and multiply in haunts all of which afford 

 a sufficient degree of shelter and an adequate and easily 

 obtained food-supply, while the conditions as regards 

 humidity allow the necessary gaseous exchanges to go on 

 through the delicate body-wall. Springtails have undergone 

 profound racial changes which may be regarded as indicating 

 degenerative specialisation. Among these is the loss — total 

 or extensive — of the typical insectan system of air-tubes for 

 breathing ; these insects have reverted to a primxitive method 

 of breathing through the general surface of the skin such as 

 is practised by the earthworms and other lowly organisms. 



Another degenerative change is the disappearance of the 

 compound eyes ; throughout the order these are replaced 

 by sets of simple eyes (ocelli) eight at most on either side of 

 the head (Fig. 66, h), and in many species there are no eyes 

 of any kind. Such blind springtails have been mentioned 

 as characteristic denizens of caves. They have often been 

 regarded as blind on account of their residence in darkness 

 through a long succession of generations ; but, on the other 

 hand, caves and other similar dark places furnish haunts 

 which are tolerated by creatures that cannot see, while 

 those with well-formed eyes generally have the reaction of 

 approaching any perceived source of light and will not 

 therefore remain in darkness if, after wandering or being 

 transported thither, they find themselves free to make a way 



