70 NORTH AMERICAN DIPTERA. 



found in decaying fungi and wood and under carpets or in 

 furniture, and are supposed to be carnivorous. The flies are 

 not very active in their habits, and because of the frequency 

 with which they are observed on window-panes are usually 

 called window-flies. 



TABLE OF GENERA. 



First posterior cell narrowed ; legs rather stout. SCENOPINUS Latreille. 



First posterior cell closed before the margin of the wing; legs slender 



(Mexico). . . . r _. . PSEUDATRICHIA Osten Sacken. 



24. ACROCERIM:. 



Small to large, never elongate, pilose or nearly bare flies. 

 Head small or very small, chiefly occupied by the large eyes, 

 which are usually contiguous in both sexes above or below, or 

 above and below the antennae; two, three or no ocelli present; 

 antennae composed of two or three joints, with or without a 

 terminal arista or style. Proboscis rudimentary or long, 

 sometimes very long. Thorax large, spherical ; tegulae very 

 large and inflated ; scutellum large. Abdomen closely united 

 to the thorax, large and inflated. Legs rather stout ; the tarsi 

 with three membranous pads under the claws. Neuration 

 variable, the veins sometimes weak and indistinct. 



This family, the Acroceridae or Cyrtidae, comprises a small 

 number of curious flies with curious habits. They are easily 

 recognizable by their small head and large, inflated tegulae. 

 No family characters can be drawn from the neuration, owing 

 to the great differences often existing between forms other- 

 wise related. In the few forms in which the larvae are known 

 they are all parasitic upon spiders or their cocoons. "In 

 the spring of 1887, while hunting for spiders, I found hanging 

 in cobwebs several soft white maggots and pupae. The webs 

 were generally old and out of repair, and a closer examination 

 showed that there no living spider was in them, but almost 

 every one had an empty skin of a common spider Amaurobius 



