248 INSECT ARTIZANS AND THEIR WORK 



may be found — there are other flies whose functions 

 are more narrowly defined — what one might term 

 sanitary specialists. Such are the Bee Flies (Volu- 

 celld) which were long thought to be parasites 

 on bees and wasps, because their larvae and pupae 

 were often found in the nests of Humble Bees 

 and wasps underground. Instead of parasites they 

 are commensals — companions who give as well as 

 receive. They are not of the highest order of 

 commensals, those that are cherished by their 

 hosts ; these appear to be only tolerated, as though 

 the nest-owners have only a dim perception of the 

 importance of the duties Volucella performs. 



In exploring wasps' nests at the end of the 

 season when the colony has practically come to an 

 end, we have found a great number of Volucella 

 larvae crawling about the combs and thrusting their 

 narrow heads into the depths of the cells. When 

 a wasp-grub comes to the winged condition and 

 quits the cell it leaves behind at the bottom (top 

 as the wasp sees it, for the cells are all inverted) a 

 thick black cake composed of the grub's excrement 

 and cast skins. It is doubtful if a wasp could 

 clean this out to make the cell fit to receive an 

 egg and to cradle another grub : the wasp's head 

 is too broad to reach the bottom. 



This is where the services of the Volucella grub 

 come in. Its body, though broad behind, narrows 

 to the front, and the head is quite small. He 

 reaches to the end of the cell and feeds on its 

 objectionable contents, making it fit for another 



