SILK-PRODUCING INSECTS. 17 



will be long or short as this is slowly or rapidly 

 excited." 



It is during the spring that the eggs of the 

 silkworm moth undergo artificial incubation or 

 hatching. This is effected by submitting the eggs 

 to a temperature ranging from 16 to 18 Cent. ; 

 but sometimes to quicken this operation the heat 

 is raised gradually to 28". The eggs are hatched in 

 ten or twelve days, when the young larvse are care- 

 fully separated from their former envelopes, and 

 reared on the leaves of the mulberry tree. 



M. Perrottet has remarked that silkworms' 

 eggs carried from France to the West Indies, and 

 kept in those hot climates for seven or eight years, 

 could not be hatched until eight or nine months had 

 elapsed, notwithstanding the high temperature, and 

 then only at long and irregular intervals. But when 

 the same eggs were put in an ice-house for four or 

 five months, they were hatched within ten days from 

 their being exposed to the circumambient atmos- 

 phere, and nearly all at once. 



In establishments where the rearing of silk- 

 worms is carried on upon a large scale', the rooms 

 ought to have a degree of warmth ranging from 

 16 to 18 Cent., and it is of the utmost importance 

 that the air of these rooms should be perfectly pure. 

 Artificial ventilation is therefore as necessary here 

 as in an hospital. M. Dumas, who has paid great 



C 



