MAY FLIES AND MIDGES OF NEW YORK 183 



platypus from Arizona and which probably also belongs to 

 this genus, making four species thus far known. 



The egigs of T. c o n g r e g a t a are laid in strings of gelatine, 

 in which the elongate oval eggs are placed. The larvae of the 

 two species of which they are known live in rapidly flowing 

 water. Here the larva spins upon the surface of the rock a 

 cocoon so loose, transparent, and open that it is not hidden by 

 it, though it prevents the larva from being washed away. The 

 larva greatly resembles Dii a m e s a from which the one Ameri- 

 can species which is known in the larval state can be distin- 

 guished by the form of its labium. The larva is pale green in 

 color with a dark brown head, and without blood-gills on the 

 ventral surface of the eleventh segment. The pupa is apparently 

 without thoracic respiratory organs ; and the dorsal surface of 

 the abdomen is provided with numerous setae. 



Imago. The genus resembles in many respects both r t h o - 

 c 1 a d i u s and D i a m e s a ; from the former it may be distin- 

 guished by its having the fourth tarsal joint of all the feet in both 

 sexes obcordate and shorter than the fifth ; from the latter genus 

 in having no M-Cu. crossvein (pl.30, fig.l2). 



Head small, broad, eyes elongate, somewhat emarginate, ocelli 

 wanting. Antennae 14-jointed in the male, long, densely haired, 

 the 14th joint like that of C h i r o n o m u s; antennae of female 

 7-jointed, sparsely short-haired, joints rounded, basal joint of 

 both male and female flattened, disk-like. Palpi 4-jointed. 

 Thorax arched. Abdomen of the male moderately slender, of the 

 female shorter and stouter. Legs raoderatelj' slender, fore meta- 

 tarsus shorter than the tibia, the fourth tarsal joint of all the 

 feet in both sexes shorter than the fifth and obcordate. Wings 

 bare, venation resembling that of O r t h o c 1 a d i ii s, the M-Cu. 

 crossvein "wanting. The forking of the cubitus may be either 

 proximad or distad of the R-M crossvein. 



KEY TO SPECIES OF THALASSOMYIA 



Imagines 



a Dorsum of thorax blackish with indications of three s-tripes; humeral 



spots, scutellum, and pleura yellowish or brownish ; abdomen dull 



black, first two segments greenish ; length 3 to 5 mm. (Neiw York) 



1. obscura 

 aa Thorax black, humeral spot yellow; length 2.5mm. (Flagstaff, Arizona) 



2. platypus 



