1913] Frost— Peculiar Habits of Small Diptera 37 



tively phototropic. Those produced during the end of the season 

 become strongly negatively phototropic and after entering into a 

 state of partial hibernation, no longer react phototropically but 

 instead become positively thigmotropic and negatively geotropic. 

 Upon the approach of spring they become positively phototropic 

 up to a low intensity and normally negatively phototropic and in 

 addition positively chemotropic to certain stimuli. Herein is in- 

 cluded the instinctive action of hibernation dependent upon or 

 induced by several tropic stimuli. 



PECULIAR HABITS OF SMALL DIPTERA, 

 DESMOMETOPA LATIPES MEIG. 



While collecting bees on the flowers of the wild aster on Septem- 

 ber 15, 1912, 1 noticed a small fly make a dash at a spider that was 

 resting on a milkweed leaf. Closer examination showed that the 

 spider was feeding on a plant bug, Lygus pratensis L., which was 

 wholly hidden beneath the body of the spider. There were also 

 three of the flies on the leaf an inch or so away, and occasionally one 

 or all of them would rush in under the forelegs of the spider and 

 cling to the body of the victim. They did not seem to mind the 

 constantly moving forelegs and were only disturbed when the 

 spider moved itself and prey bodily. Possibly they had to move 

 to escape being jammed against the leaf. I was not able to make 

 sure that they were lapping up the juices of the bug but I have no 

 doubt that this is the reason for their actions. 



The fearlessness of these small insects, barely equal to the head 

 of the spider, was astonishing to me and I can but wonder at the 

 acquisition of such a habit of robbery from so formidable an 

 enemy. 



All the specimens were captured and submitted to Mr. C. W. 

 Johnson who very kindly furnished the names of them. The 

 spider is a female Phidippus multiformis Emerton, and the flies 

 are Desmometopa latipes Meigen. 



C. A. Frost. 



