164 THE RATE OF GROWTH [ch. 



morphoses of an insect are but phases in a process of growth was 

 clearly recognised by Swammerdam, in the Bihlid Naturae*. 



A stick-insect (Dexippus) moults six or seven times in as many 

 months ; it lengthens at every ilioult, and keeps of the same length 

 until the next. Weight is gained more evenly; but before each 

 moult the creature stops feeding for a day or two, and a little weight 

 is lost in the casting pf the skin. After its last moult the stick- 

 insect puts on more weight for a while; but growth soon draws to 

 an end, and the bodily energies turn towards reproduction. 



We have careful measurements of the locust from moult to moult, 

 and know from these the relative growth-rates of its parts, though 

 we cannot plot these dimensions against time. Unlike the meta- 

 morphosis of the silkworm, the locust passes through five larval 

 stages (or "instars") all much ahke, ' until in a final moult the 

 "hoppers" become winged. Here are three sets of measurements, 

 of Hmbs and head, from stage to stage f. 



Growth of locust, from one moult to another 



Length (mm.) Percentage-grpwth Ratios 



As a matter of fact the several parts tend to grow, for a time, at 

 a steady rate of compound interest, which rate is not identical for 

 head and hmbs, and tends in each case to fall off in the final moult, 

 when material has to be found for the wings. Some fifty years ago, 

 W. K. Brooks found the larva of a certain crab (Squilla) increasing 

 at each moult by a quarter of its own length ; and soon after 

 H. G. Dyar declared that caterpillars grow hkewise, from moult to 

 moult, by geometrical progression % . This tendency to a compoimd- 



* 1737, pp. 6, 579, etc. 



t A. J. Duarte, Growth of the migratory locust. Bull. Ent. Res. xxix, pp. 425-456, 

 1938. 



% W. K. Brooks, Challenger Report on the Stomatopoda, 1886; H. G. Dyar; 

 Number of moult^ in lepidopterous larvae, Psyche, v, p. 424, 1896. 



