METAZOA INSECTA. 395 



spring, many of the Poduridae have a sucker attached to 

 the basal portion of the abdomen (see PL 952), by means 

 of which the insect attaches itself. According to Uljanin 

 quoted by Cholodkowsky ! the springing-fork of the Pod- 

 uridae arises from two abdominal appendages which are 

 in every respect similar to legs, so that their homology 

 with the thoracic limbs is hardly open to doubt. The 

 ventral tube also develops from two anterior abdominal 

 appendages \thich are probably homologous with the 

 thoracic legs. The tiny leapers that sometimes blacken 

 large patches of our snow, making it appear as if cov- 

 ered with animated coal dust, are spring-tails (PI. 953, 

 Achorutes ?iivicola Fitch, possibly unnaturally swollen). 

 It is surprising to watch the leaps of these minute ani- 

 mals, sometimes covering several feet (Comstock) and 

 again jumping many times their height into the air. 

 Although bluish black when full grown, they are white 

 when young. 2 



There are other Poduridae which have a spring in the 

 larval stage but which lose it on reaching maturity. One 

 of the springless forms is Lipura maritima Guer. (PI. 

 954). This is one of the few insects that adapt them- 

 selves, for short periods at least, to salt water. It is found 

 on the surface of tide pools and immersed in the water, 

 but when in this situation it is said to be protected by a 

 layer of air that envelops its body. 8 



We cannot but think that some of the primitive wing- 

 less ancestors of insects, which in all probability were 

 essentially like the Thysanura, developed in the course 

 of many generations the wing sacs or pads that finally 

 became efficient organs of flight. It is true, as before 

 stated, that so far winged insects of as primitive a nature 

 as the Thysanura have not been found in a fossil state, 

 but it may be that still older rocks than the Devonian 



1 Ann. and Mag. Nat. Hist., (6), X, 1892, p. 434. 



2 Packard, 5th Ann. Rep. Peabody Acad. Sci., 1872, p. 30. 



3 Cambridge Nat. Hist., V, 1895, P- J 95- 



