GROWTH OF LARVAE. 193 



nnii n. Dr. Harris, in his memoir upon the salt-marsh cater- 

 pillar, also notices that there are two varieties of the moth, 

 corresponding with the caterpillars from which they are pro- 

 duced ; from the dark caterpillar and brown cocoon proceeds 

 a moth with ash-coloured wings, and from the lighter coloured 

 larva and cocoon is disclosed a moth, whose upper, and also 

 sometimes the lower, wings are white, these colours not 

 designating the sex. De Geer also observed, that the brown 

 caterpillars of the yellow underwing moth (Triphcena pro- 

 nuba) produce males, and the green ones females. 



Respecting the size and growth of larvae, it is to be ob- 

 served as a general rule, that they are longer and heavier, 

 when full grown, than the perfect insect. According to the 

 Count Dandolo, the following is a statement of the pro- 

 gressive increase in the weight of silkworms : 



Grains 



A hundred worms, just hatched, weigh about . 1 

 After the first moulting . . .15 



After the second moulting . 94 



After the third moulting . . .400 



After the fourth moulting . . . 1628 



On attaining the largest size . . 9500 



From what has already been advanced in my observations 

 upon the principles of metamorphosis, it will readily be 

 conceived, that with this rapid growth a continued moult- 

 ing must be required. Hence we find caterpillars are sub- 

 ject to a greater or less number of these changes of the 

 skin; the great tiger-moth (Arctia caja) undergoing as 

 many as ten such moultings, Arctia dominula nine, Arctia 

 villica from five to eight. The ordinary number, however, 

 appears to be three or four, but it would seem that the 

 grub of the bee, and many other footless grubs, undergo no 

 moulting, although both Reaumur and Swammerdam assert 

 that the contrary is the case. I have already, in my ob- 



