46 THE SIX-FOOTED INSECTS 



be arranged in clusters on the surface, attached to objects 

 above the surface, to leaves overhanging the water, to stems 

 of aquatic plants, either above or below the water line, and 

 for some of the distinctly aquatic forms placed upon the 

 bodies of the insects themselves. The period of incubation 

 varies enormously, some hatching immediately upon deposi- 

 tion, or, in some cases, preceding deposition, in which 

 case the insect appears to be viviparous and for the other 

 extreme remaining in the egg stage for many months, many 

 species passing the winter in this stage. Ordinarily the 

 eggs of any particular egg mass, or of any species, hatch 

 with great uniformity, so that larvse will appear at the 

 same time. This results at times in the very sudden appear- 

 ance of larvae in startling numbers and to the iminitiated 

 suggests the occurrence of some very remarkable invasion. 

 Usually no nutritive material, other than contents of the 

 egg, can be used during this period, but some species in 

 which the eggs are forced into plant tissues and in which 

 the egg covering must be very delicate, there is an absorp- 

 tion of fluids indicated by the distinct increase of the size 

 of the egg prior to hatching The hatching of the egg is 

 usually accomplished simply by pushing off of the egg-cap 

 or rupture of the egg membrane, but in some species it 

 depends upon external factors associated with the future 

 history of the larva. For instance, the eggs of the horse 

 bot fly are hatched only upon the application of friction 

 or moisture and warmth, conditions which are brought 

 about when the horse licks the hair bearing the eggs and 

 thus provides ready means of transfer from the eggshell 

 to its mouth, thus providing the necessary conditions for 

 future development of the insect. The particular method 

 of hatching may therefore have very important relation 

 to preventive or remedial measures. 



The larval stage is the active feefling stage during which 

 the growth of the insect occurs, and during this period there 

 are a varying number of moults, most frequently from four 

 to five, at which there is a rapid increase in size, the larva 

 accommodating itself to the tough, chitinous body wall, 

 which as soon as hardened is incapable of any expansion. 



