1 84 



NORTH AMERICAN DIPTERA. 



ing between forms otherwise related. In the few forms 

 in which the larvae are known they are 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 no living spider was in them, but almost every one 

 had an empty skin of a common spider Amaurobius sylves- 

 tris, nearly full grown. The skin of the legs and thorax 

 was not clean like a moulted skin, but dirty and opaque, 

 as though eaten out, and the skin of the abdomen when 

 present was torn and shriveled. From this I concluded 

 that the maggots came out of the spiders, and from their 

 size must have nearly filled them. The maggots varied 

 considerably in size, the largest being a quarter of an 

 inch long, while others were not more than half as large. 

 The hinder half cf the body was thicker than the front 

 half and nearly spherical. They hung head downward, 

 holding to the web by their jaws and were also partly 

 supported by threads under and around them.' The 

 author of the foregoing, J. H. Emerton, reared from other 

 specimens of these larvae a fly belonging to the genus 

 Acrocera. The larvae of Astomella lindclii, according to 

 Brauer, are so lodged in the abdomen of the spider that 

 the posterior terminal stigmata are in relation with the 

 lung-tubes of the spider. The eggs are said to be de- 

 posited on dried twigs. 



TABLE OF GENERA. 



1. Antennae without terminal arista or style 2 



Antennae with a terminal arista 3 



2. Antennae short, third joint rounded, with terminal bristly hairs. 3 

 Antennae elongate. 7 



3. Antennae inserted below the middle of the head in profile. . 4 

 Antennae inserted above the middle of the head. ... 6 



