1314. Why do we see, in tanks of rain water, insects rising to the surface?

Because numerous insects pass through their first stages of existence in water, and among them the common gnat. The gnats of the previous season having deposited their eggs on the sides of the water-butt, the warm water developes them, and the larvæ of the gnats appear ([Fig. 81]; c natural size of larva; b larva magnified).

Fig. 81.—LARVA AND PUPA OF GNAT.
(Greatly magnified.)

1315. Why do they continually rise to the surface of the water?

Because they require to breathe air, and therefore they come up to the surface, where, elevating the tube (b) above the surface of the water, they are enabled to breathe.

1316. Why do some appear to have larger heads than others?

Those that have apparently larger heads, and that breathe through tubes attached to their heads (d) are in the pupa, or second stage of development, and underneath the large shield by which their heads are marked, their wings, feet, &c., are being formed.