Bombycidae 363 



which they crawl and begin to feed. Only young, tender mulberry 

 leaves should be used for the small caterpillars. In all cases clean, healthy 

 leaves should be selected, and it is usually well to wash the leaves. The 

 mulberry leaf with the young larvae on it is transferred to a shallow tray, 

 about 6 inches square with sides i%-inches high, made out of previously 

 autoclaved cardboard. No cover is needed. Fresh mulberry leaves are 

 supplied and the trays cleaned at least once a day. The young larvae 

 are not handled but are removed together with the leaves to which they 

 cling. The excreta and dry leaves are shaken out of the tray. The leaves 

 with the larvae are replaced and covered with a few fresh leaves, to which 

 the larvae crawl. Caterpillars which must be handled directly are best 

 taken up with a camel's hair brush. The larval stage of the silkworm 

 consumes from 4 to 6 or 7 weeks, depending on the temperature, and con- 

 sists of 5 instars. When a group of about 50 larvae enters the 3rd instar 

 it should be transferred to a cardboard tray about 1 2 inches square with 

 sides 3 inches high. Larvae in the 4th and 5th instars are large and may 

 be handled directly. Larvae entering the 5th instar should be thinned 

 out to not more than 20 to 30 to a large tray. 



When one is working with a silkworm disease, the healthy stock larvae 

 should be kept in a separate room in which no diseased material is pres- 

 ent. They should be fed in the morning before diseased material has been 

 handled. If it is necessary to go to them again later, one's hands should 

 first be thoroughly washed. Observation of these simple precautions pre- 

 vents infection of the stock from the diseased experimental worms. The 

 washing of the leaves and autoclaving of the cardboard for the trays re- 

 duce the chances of infection from external sources. If an occasional 

 caterpillar does get sick or dies, it should immediately be removed. If 

 a number of larvae in one tray die, this tray with all the larvae in it 

 should be discarded. 



For careful studies of disease, the experimental worms should be kept 

 singly in % pint bottles, previously capped with paper and sterilized 

 by dry heat. When it is desired to feed known quantities of infective 

 material with a pipette, the worms (preferably in the 5th instar) should 

 be starved for one day. 



The stock may be perpetuated by allowing the full grown worms not 

 needed for experiments to spin cocoons. The cocoons are conveniently 

 kept in large cardboard trays. The moths, which emerge about 2 weeks 

 after spinning of the cocoon, mate readily, and each pair should be placed 

 on a large sheet of cardboard under a % pint bottle or a small lamp 

 chimney. Eggs laid by the female moths adhere to the cardboard, which 

 may later be cut into squares, each square holding the eggs laid by one 

 moth. The newly laid eggs are yellow but, unless they are sterile, they 

 gradually darken and become gray within 7 to 10 days. When the eggs 



