THE BIOLOGY OF THE MUD-DAUBING WASPS 37 



larva just before it reaches its prepupal stage. Even as late 

 as June 1, one often finds upon opening the mud nests, these 

 Diptera larvae which so closely resemble the wasp larvae as 

 to be at first glance deceiving. When the time for emerging 

 arrives this pupa works itself out through the mud wall until 

 it protrudes half way; then the pupal case breaks and liberates 

 the mature, winged insect, the old shedding skin still blocking 

 up the hole made in the mud wall. 



Sharp 11 says that the life history of these insects was unknown 

 until Fabre ascertained that another belonging to this genus, 

 A. trifasciata, was parasitic upon the mason-bee. This fly, 

 according to Fabre, oviposits by dropping a minute egg on the 

 mass of masonry by which the grubs of the mason-bee are 

 protected. From this egg is hatched a minute larva, which is 

 provided with a horny head and bristles serving as organs of 

 locomotion ; thus equipped it explores the surface of the masonry 

 for a long time seeking an entrance. Having penetrated the 

 wall, it undergoes a complete change of form, and without 

 inflicting any perceptible wound upon the pupa it sucks day 

 after day until it has completely emptied its contents. For 

 some months it remains a quiescent larva in the cells of the 

 mason-bee, but in the spring it undergoes another metamor- 

 phosis and appears as a pupa, provided with formidable weapons 

 for breaking down the masonry by which it is imprisoned. It 

 strikes against the opposing wall until a gallery of exit is formed; 

 then the pupal skin bursts and the perfect fly emerges leaving 

 the pupal case still fixed in the gallery. 



These details describing the European species apply equally 

 accurately to our A. obsoletum. 



Other Diptera which emerge from the mud-daubers' nests 

 although less frequently, are Toxophora americana Guerin 12 and 

 Spogostylum fur O. S. 12 



In addition to these two species of Diptera one finds old 

 pupal cases of flies of a smaller species, in the old remains of 

 spiders, etc. in the cells, but I have never yet been able to obtain 

 adults therefrom. There are sometimes from three to a dozen 

 in a single cell. 



Birds, while they are neither parasites nor inquilines, should 



11 Cambridge Nat. Hist., Pt. II, p. 486. 



12 Identified by Mr. F. Knab. 



