364 NATURAL HISTORY OF AQUATIC INSECTS CH. 



and his seventh and last volume appeared shortly 

 afterwards in the same year. This last volume gives 

 his views upon the classification of Insects. He 

 regarded the characters drawn from the wings as of 

 primary importance, but attended also to the structure 

 of the mouth-parts and to the mode of transformation. 

 His system was on the whole the best of its century. 



The first paper ever published by De Geer l was 

 written at the age of twenty. In it he describes four 

 small Insects belonging to what is now called the 

 order of Thysanura. 2 The Thysanura are all of small 

 size and wingless ; they undergo no transformation ; 

 many leap about by means of a forked appendage 

 attached near the end of the abdomen to its under- 

 side, and are hence called " Spring-tails." Leeuwen- 

 hoeck had previously found and described Thysanura. 

 but De Geer's account is fuller and more exact. Of 

 his four species two, now called Podura aquatica and 

 Isotoma palustris, were found on the surface of water, 

 De Geer's account of Podura aquatica is here quoted 

 in brief. 



In the month of February he observed good- sized 

 black patches floating on small pools of water. 

 Closer examination showed that the patches were 

 made up of small Insects clustering together and 

 moving incessantly. Though resting on the water, 

 they were never wetted. When touched with a stick 

 the Insects dispersed, but gathered together again as 



1 Ada Soc. Reg. Scient. Upsal, Vol. I. (1743), p. 279. The 

 paper was read in 1740. 



2 Seven species are described by De Geer in the last volume 

 of his Memoires (1778). 



