206 [August 



the water nor out of the water, did I ever observe the antennae to be 

 diseno;aa;ed from the lower surface of the breast and thrust forwards. 

 They crawl quite slowly, but swim very rapidly, the caudal set?e being 

 the chief organ of motion in swimming, and the legs being only used 

 occasionally to direct their coui-se. When swimming the caudal setje 

 and the tip of the abdomen are directed upwards and backwards at an 

 angle of 45° with the body, and with this as the axis of oscillation 

 are vibrated vigorously and rapidly up and down. When desirous of 

 rapidly changing their course, they have the power of elevating the tip 

 of the abdomen and setae so as to lie along the dorsum of the shield, 

 and then suddenly lashing out with them. I noticed that in repose 

 they sometimes adhered to the under surface of the floating cork for 

 hours together, and sometimes to its side, so that their body would be 

 half in and half out of the water. When they were taken out of 

 the water the tip of the notal shield generally after a few seconds 

 gaped apart from the fifth dorsal joint of the abdomen, and the pal- 

 pitations and structure of the branchife became plainly visible under 

 the shield. The reason is obvious. They were then compelled to 

 breathe through their spiracles, instead of through their branchiae. 



The pupa crawls out of the water to assume the subimago state, 

 which process is performed by the notal shield splitting open dorsally 

 in a straight line from end to end, and the head being disengaged back- 

 wards from the pupal head without splitting it. Described from S 

 specimens, some living, some alcoholic, procured in the Mississippi 

 Rapids, June 5 — 11; the first subimago appeared June 13, from a 

 specimen obtained June 5. Length % (one specimen) 7 millimetres. 

 9 (one specimen) 7 2 mill. Breadth % 0-4- mill. 9 4 mill. Setae % 

 21 mill. 9 2J mill. 



Rock Island, Illinois, July 1, 1864. 



