274 [December, 



southern end of Denny Bog. Here we were soon busily engaged in 

 watching the burrows of various species of Fossores, our object 

 being to ascertain the kind of prey that was stored for the future 

 progeny. I was thus engaged when my friend, who had been 

 prospecting a little further afield, returned and reported the dis- 

 covery of a large colony of Cerceris arenaria^ a species I expressly 

 wished to find. While watching the burrows we noticed a fly, some- 

 what like a Miltogramma, persistently following a female Cerceris 

 returning to the burrows with her weevil prey. We felt sure that 

 the Fossor was aware of the pursuit, and that she endeavoured to 

 some extent to shake off her enemy, for, instead of flying direct to 

 the burrow, she often dodged or zigzagged about, or settled upon the 

 ground. These tactics were of no avail, for the fly followed all the 

 meanderings with perfect ease, always keeping at a distance of about 

 three or four inches behind. This we saw many times repeated, and 

 always with the same result. The observations were continued with 

 a view of ascertaining the further actions of the fly. It was found 

 that when the Cerceris entered the burrow with her prey, the fly im- 

 mediately settled close by the hole and remained perfectly motionless 

 — often for as long as five or six minutes — facing the orifice. When 

 finally the Fossor departed in quest of further prey, the fly entered 

 the burrow, probably in order to oviposit, re-appearing again in less 

 than a minute. Although males of Cerceris nrenaria were flying 

 about the burrows, in no case did we observe that the fly followed 

 them : only the females with prey were pursued, sometimes by two 

 and in one instance by three of the Dipteron. 



The seven individuals mentioned by Mr. Colbran J. Wainwright 

 were the specimens actually captured following the Cerceris with prey, 

 and in each case fly, Fossor and prey were taken with a .-liugle stroke 

 of the net. Many others were seen, and some captured as they sat 

 upon the bare sandy ground. 



On August 10th we found another and much smaller colony of 

 Cerceris arenaria, near Matley Bog, when we again observed the fly 

 behaving exactly as on the former occasion. 



My sincere thanks are due to Mr. Colbran J. Wainwright for 

 kindly identifying and writing the following account of Setulia 

 grisea, Mg. Examples of specimens described by him may be 

 seen by entomologists in the Hope Department, Oxford University 

 Museum. 



University Museum, Oxford : 

 November, 1909. 



