64 WHITE FLIES INJURIOUS TO CITRUS IN FLORIDA. 
Pronounced and striking growth in size occurs only at molting, 
when the soft flexible skin of the larva or pupa is able to stretch before 
assuming its normal rigid condition. With each successive molt the 
larva greatly increases its horizontal dimensions, until by the time it 
reaches the pupal stage these are about eighteen times as great as in the 
newly hatched larva. When first settled after molting the larva is very 
thin, papery, and transparent, being seen with difficulty except with 
the aid of a lens, but after feeding several days it slowly becomes 
thickened until, from two to five days, sometimes longer, before molt- 
ing into the next instar, it is decidedly plump and whitish opaque in 
color. Oftentimes before molting the larva becomes very much 
swollen as though gorged with liquid. This appears to be an abnor- 
mal condition, since many that become thus unduly enlarged either 
fall or die without molting. During the increase in thickness follow- 
ing feeding, there is no increase in the horizontal dimensions. On 
the contrary, increase in the former is secured at a slight expense of 
the latter. 
Daily observations on over 300 marked individuals from time of 
settling to emergence of adult have conclusively demonstrated that the 
larva passes through but three instars 1 before reaching the pupal 
stage, instead of four as has been previously supposed. Each larva, 
then, molts or casts its skin three times before becoming a pupa. The 
process of molting was first described by Riley and Howard 2 and as 
observed by the authors is as follows : 
In preparing for a molt the insect curves the abdomen upwards at considerably 
more than a right angle, moving it also occasionally up and down. The margin of 
the abdomen has at the same time a slightly undulating motion. During these move- 
ments the insect is shrinking away from the lateral margin until it eventually occupies 
only about one-third of the original lateral space, causing a distinct dorsal and ventral 
median ridge. The skin then splits, not on the dorsum, as would be expected, but 
either at the anterior end or underneath the head. The head and prothorax are then 
pushed out and the skin is gradually worked backwards by means of the abdominal 
motions, the portion already out swelling as soon as it is free. 
As the insect flattens after molting it appears milky white, the head, 
thoracic lobes, and abdominal segments being more greenish. At 
this time the legs, which resemble much the prolegs of a caterpillar, 
are very active, and there appears a pair of fleshy protuberances more 
or less movable, not as large as the legs, but apparently of the same 
1 This agrees with the senior author's observations on the greenhouse white fly 
(A. vaporariorum) and the strawberry white fly (A . packardi) , which are the two species 
of the genus which have previously been studied in greatest detail. Tech. Bui. 1, 
Mass. Exp. Sta. and Can. Ent., vol. 35, pp. 25-35. 
2 Insect Life, vol. 5, p. 223, 1893. 
