480 



OTHER BLOOD-SUCKING FLIES 



e/e 



__p ro |. 



first segment (Fig. 222 prol.) is used for creeping, in conjunction 

 with the posterior sucker, the larva looping along like a " meas- 

 uring worm"; it is also of use in constructing the silken cocoon 

 from the secretions of the salivary glands. This single little 

 leg has a crown of tiny booklets which make it possible for the 

 possessor to hold its ground even in a torrent of water. The 



salivary glands referred to are quite 

 unlike those of other insects, in that 

 they extend clear back to the pos- 

 terior end of the body (Fig. 222, sal. 

 gl.). The fluid secreted hardens to 

 silk at once on exposure to water, 

 and is used not only in spinning the 

 cocoon, but also in spinning anchoring 

 threads and life-lines. According to 

 Malloch, the larva when disturbed 

 releases its hold and floats downstream, 

 holding by the stumpy leg to a silken 

 thread which is being spun out, and 

 by means of which the insect later 

 regains its former position. The 

 larvae breathe by means of tiny gills 

 which can be projected through a slit 

 in the last segment of the abdomen 

 (Figs. 221 and 222, an. g.). The larvae 



FIG. 222. Larva of black- are never found solitary, as would be 

 fly, Simuiium venustum, side expected from the manner of laying 

 JT^rrr : eggs; to* author ^ seen the boards 



dig. tr., digestive tract; m. f., on the bottom of a log chute COm- 



SS fa and : n^t Pletdy covered with mosslike patches 

 posterior sucker; sal. gl., saii- of these larvae for areas of a square 



vary and spinning gland. yard Qr 



on.gr 



sal. gt. 



-di^.tr. 



post. 3. 



After four or five weeks, in summer, the larvae prepare to go 

 into the resting pupal stage, and spin for themselves a partial 

 cocoon which is variously shaped like a jelly glass, slipper, wall 

 pocket, etc., open at the upper end for the extrusion of the 

 branching gill filaments which are used as breathing organs (Fig. 

 221C). Some species, simply spin a snarl of threads, the work 

 of a whole community, in the meshes of which the pupae exist in 

 a fair state of protection. The general form of the pupae can be 



