126 Journal New York Entomological Society. t^oi. xx. 



variation may occur, at least under ordinary insectary conditions, 

 although five moults (six stages) is no doubt normal. 



Single larvae were isolated in glass battery jars in the insectary 

 and fed on cherry leaves. A small cherry twig was placed in a bottle 

 of water in each jar. Since there was no danger of the slugs escaping 

 from the jars, these were usually left open at the top, although some- 

 times covered with netting. Observations were made daily on these 

 cages, all moults recorded, and the cast head measured in every case, 

 In moulting this species leaves the head intact, not splitting it down 

 the middle, as do some saw-fly larv?e. It is the habit of the larvse 

 to eat their cast skins, although the head itself it not eaten, but 

 remains on the leaf. By cutting out a small piece of leaf together 

 with the cast head, this could be placed on a microscope stage and 

 the head easily measured. The measurements were made with an 

 ocular micrometer, used with a Leitz compound microscope. No. 3 

 objective. No. 2 ocular. 



In the accompanying table of head widths of individual larvje all 

 the available data were used, whether or not a larva became full grown. 

 Consequently a part of the records are incomplete, but all the figures 

 given here were used in computing the average head width for the 

 several stages. Each vertical column in the table represents a single 

 larva, and each figure a separate and distinct moult. 



It will be noticed that where complete records are given, larvae 

 moulted from five to eight times. Of these, five larvae moulted 5 

 times, five 6 times, three 7 times and i larva moulted 8 times. 



Considering first those larvae that moulted five times it may be 

 noticed that with only two of these the head width increased regu- 

 larly in anything like a geometrical proportion. These are the larvae 

 in cages 53/09 and 214/10. In general the head width of a larva 

 increases at some definite geometrical ratio with each stage, a fact 

 to which Dr. Dyar (1890) has already called attention. In the case 

 of the pear-slug the ratio of increase is practically .35. 



In the other three cases there has been a slip in the progression. 

 In cage 70/09 stage IV seems to have been missed entirely, for the 

 width .^2 mm. is hardly enough to give it a place in that stage. In 

 cage 421/10 stage IV is clearly missing, even though there are five 

 moults. Here the larva moulted twice, and the cast head measured 

 .65 mm. each time. When it next moulted the head was 1.08 mm. 



