282 MR. F. BALFOUR-BROWNE ON THE [Mar. 2 
assuming that there are ectodermal developments to take place, 
it seems to me that so long as those developments do not take 
place in stages of excessive length, development will always be 
the contr olling factor. 
From the time of the moult the new ectoderm of the nymph 1s 
steadily hardening (probably oxidizing), and gradually losing its 
elasticity. As soon as the skin has ceased to expand with the 
growth of the nymph, the pressure on the body of the nymph 
reacts so that a new layer of chitin is laid down within, and 
separate from, the old layer. Once this new layer has completely 
isolated the old one the latter becomes brittle, and gives way 
along a line of weakness in the median line of the thorax when 
the nymph exerts itself and escapes. Such would be the explana- 
tion of a ‘‘growth-moult” pure and simple. If, however. 
previous to this condition being reached a developmental stage 
had been arrived at, a moult will take place which will serve 
both purposes, but will have been caused by developmental 
requirements. 
In the case of one of my nymphs two moults took place within 
4 days of each other. The second moult could not have been 
required for growth nor, on the excretion theory, for excretory 
pu pOSes, and I imagine that the first moult was a “ growth- 
moult ” and the second a “developmental” one, especially as the 
first moult was the first to take place after the nymph had been 
put into the incubator, and 17 days after the previous one. 
In this connection the effects of temperature on the number of 
moults are important. Packard (/. c¢. p. 616) refers to the 
experiments of Weniger and others on the larve of certain 
Lepidoptera, the results of which show that under warm moist 
conditions the number of moults is reduced from 5 to 4; and 
referring to certain other experiments, he says (p. 618) : 7 ‘As to 
the cause of the great number of moults in the arctians and in 
the beetles exper mented with by Riley, it would seem that cold 
and lack of food during hibernation were the agents in arctians, 
and starvation or the lack of food in the case of “the beetles, such 
cause preventing growth, though the hypodermis-cells retained 
their activity.” W.H. Edwards states that larvee of Lepidoptera 
which hibernate moult more often than those which have only a 
summer existence, and larvee with a wide geographical distribu- 
tion moult more often in cold than in warm regions. He also 
says “there seems to be a necessity with hibernators of getting 
rid of the rigid skin in which the larva has passed the winter ; 
that is, if hibernation has taken place during the middle stages, 
as it does in Apatura and Limenitis. In these cases, very little 
food is taken between the moult which precedes hibernation and 
the one which follows it, and the larva, while in lethargy, is 
actually smaller than before the next previous moult. The skin 
shrinks, and has to be cast off before the awakened larva can 
grow” (quoted from Packard, /. ¢. p. 615). 
