286 



AN INTRODUCTION TO ENTOMOLOGY 



The smoky alder-fly, 5iafo injumdta. — This is a small insect hav- 

 ing a wing-expanse of about 2 5 mm. ; the males are sometimes smaller 

 than this, and the females slightly larger. It is dusky brownish in 

 color. It can be easily recognized by the form and venation of its 

 wings (Fig. 318). 



The costal area of the fore wings is greatly expanded before the 

 middle, and most of the wing-veins are stout. A striking feature of 

 these wings, one that is characteristic of the subfamily Sialinae, is 

 that the radial sector is nearly typical in form; the only modification 

 being the development of one or more marginal accessory veins upon 

 it. These accessory veins, however, are in a quite different position 

 from that occupied by the accessory veins borne by the radial sector 

 in the Corydalinse, where a pectinately branched 

 radial sector has been developed. 



The larva (Fig. 319) is furnished with the paired 

 lateral filaments characteristic of the larvce of the 

 Sialidas on the first seven abdominal segments. These 

 filaments are more or less distinctly five-segmented. 

 The last abdominal segment is prolonged into a taper- 

 ing lash-like filament. 



The larvas are found in swiftly flowing streams 

 adhering to the lower side of stones in the bed of the 

 streams and in trashy places filled with aquatic plants 

 in the borders of streams and ponds; they are very 

 active. The larvae transform in earthen cells at some 

 little distance from the water. Two or three weeks 

 after the making of the pupal cell the adult fly 

 emerges. 



The eggs are laid in patches, each consisting 

 of a single layer of eggs. The females frequently 

 add their eggs to patches of eggs that have been laid 

 by other females. The eggs when first laid are 

 lighter in color than later. 



Several specific names have been given to what 

 are now believed to be merely varieties of this 

 species. 

 Protoslalis americana. — In this species the costal area of the fore 

 wings is only slightly expanded before the middle; and the wing- 

 veins are not as stout as in Sialis. The early stages have not been 

 described. 



Fig. 319. — Larva 

 of Sialis inju- 

 mata. (After 

 Needham.) 



Subfamily CORYDALIN^ 



Corydalus and the Fish-Flies 



The subfamily Corydallnas is represented in this country by the 

 well-known homed coiydalus and several smaller species, commonly 

 known as the fish-flies. In these insects there are three ocelli; the 

 fourth tarsal segment is not bilobed; and the radial sector is pec- 

 tinately branched (Fig. 320). The larvae are distinguished by the 



