84 INSECT PHYSIOLOGY 



to itself. But upon this general background are superimposed some 

 special phenomena, conditioned by the make-up of the insect; and it 

 is in these that we are mainly interested. In the first place, the cuticle 

 is so constructed that once formed it cannot grow in surface area. If 

 it is soft and much folded it may stretch gradually, and then the in- 

 sect may increase continuously in length as it does in weight. This 

 happens in the larvae of flies (e.g. Drosophila) and Lepidoptera (e.g. 

 Galleria); but even in these insects there are hard regions of the 

 cuticle which can grow only by replacement at the time of moulting. 

 The growth of such parts is, therefore, discontinuous; and when the 

 greater part of the insect is encased in a rigid cuticle, the growth in 

 length of the whole body is likewise discontinuous, and takes place in 

 a series of steps. 



It was observed long ago by Dyar, working with the larvae of 

 Lepidoptera, that the linear dimensions of the hard parts increased at 

 each moult by a constant ratio, usually of the order of 1-4 (Dyar's 

 law). More recently, it has been shown that various hemimetabolous 

 insects {Sphodromantis, Carausius) approximately double in weight 

 from one moult to the next, while their linear dimensions increase 

 1-26 (i.e. 3 V2) times. This growth factor of 2 was first demonstrated 

 by Przibram and Megusar; it is perhaps best regarded simply as a 

 special case of Dyar's law; but it is sometimes referred to as 'Pri- 

 bram's factor'. The physiological basis proposed by Przibram to 

 explain his factor was that at each moult (or at each latent division) 

 every cell in the insect body divides into two. But what histological 

 evidence exists is definitely opposed to this improbable hypothesis. 

 For in the Muscidae, almost the whole of larval growth is accom- 

 plished by increase in size of the cells without multiplication, and in 

 hemimetabolous insects there is much cellular breakdown and re- 

 construction besides the irregularly distributed cell divisions. 



We have seen that at the time of moulting, the insect enlarges its 

 surface area by swallowing air or water (p. 10). In the latter case 

 (Aeschna, Notonecta, &c.) the resulting curve for growth in weight is 

 peculiar and shows an abrupt rise immediately after moulting, 

 followed by a more or less level period during which the ingested 

 water is gradually incorporated as true growth (Fig. 12, B). Equally 

 curious is the growth curve of the blood-sucking bugs (Cimex, 



