The Hump-backed Spider-Fly. 



Photo by] [E. Step, F.L.S. 



The Hump-backed Spider-Fly. 



This fly lays its eggs on the egg-cocoons of spiders, 

 and the grubs feed upon the spider's eggs and the 

 voung spiders. It is thought that it may complete 

 its larval life in the body of a spider of larger 

 growth. The photograph is six times the actual 

 size. 



has been inflated by some diseased condition. 

 This effect is intensified by the fact that the 

 fore-body is covered by a rather matted coatin^i;- 

 of whitish-grey hairs, which gives it the appear- 

 ance of the house-fly that has been killed on our 

 windows in autumn by the activity of the fly- 

 fungus. It is just possible that it may not be so 

 rare as supposed, but that specimens may be 

 passed by with a cursory glance, in the belief 

 that they are merely diseased house-flies. 



The grub of this fly lives either in the interior 

 of various spiders as a parasite, or in their egg- 

 cocoons, where it feeds upon the eggs. Now, with 

 such an easy-going mode of life under cover, and 

 with a world full of spiders to feed upon, one 

 might expect these flies to be quite common 

 things ; but the fly in turn has an enemy, who 

 takes steps to keep down its numbers. This is one of the little wood-wasps, ^ 

 Insects that make tunnels in decaying wood, bramble-stems, and the like, dividing 

 the tunnels off into cells, in each of which an eg^ is placed with a number of stung 

 two-winged flies. Each species of wood-wasp appears to have its own special taste 

 in flies, and will provision its cells only with one species of fly. Whether this 

 hump-backed spider-fly is regularly exploited for the purpose is not established, 

 but that it is so used at times is made clear by the Rev. H. S. Gorham, who 

 came upon a hoard of flies in a nest of tlie wasp. The wasp, with that regard 

 for economy of labour that is shown by many species, instead of making a 

 tunnel for itself, had found one that had been excavated by the caterpillar 

 of the frosted orange-moth- in the stem of the marsh-thistle. This she had 

 divided into cells, and packed them all with the hump-backed spider-fly. 

 There were twenty-five to thirty flics in each cell, and the row of cells 

 measured eight inches. In some of the cells the wasp-egg had hatched, and 

 the wasp-grub was already feeding uj^oii the stored-up flies. 



A further peculiar circumstance of this 

 ' discovery is that Mr. Gorham's attention 

 was attracted to the thistle, not by the entry 

 of the wasp with its prey, but by the fact 

 that several spiders appeared to be watching 

 the hole in the stem. The spider — 

 whose name he did not know — so greatly 

 resembled the fly that he considered it to 

 bo a case of mimicry. This would lead one 

 to suppose that it was one of the crab- 

 spiders ,3 which arc in the habit of sitting 

 still in or about flowers visited by flies. 

 Mr. Gorham considered these spiders to be 



^ Thomisiis. 



Photo by] [E. Stt-p, F.E.S. 



The Hump-backku Spider-Fly. 



The insect is here seen from the side, a view which shows its 

 hump-backed appearance more clearly. The depressed posi- 

 tion of the head is <\u\U- natural in this species. 



1 Crabro interruptus. 



Gortyna ochracca. 



