414 THE BIOLOGY OF INSECTS 



bird's blood along with the gnat's saliva, and protective 

 spores are therefore needless. 



Such spores are, however, a conspicuous feature in the 

 life-history of some species of Nosema, minute sporozoan 

 parasites, which belong not to the Haemosporidia but to 

 another order, the Microsporidia ; two of these have attracted 

 attention, as they infest respectively two insects domesticated 

 by man — the Hive-bee and the Common Silkworm. 

 Nosema apis (Fig. 84, a) is found in its various phases in 

 the digestive tract of the Hive-bee and of other Hymeno- 

 ptera, very minute cells, young individuals of Nosema, lying 

 in the protoplasm of the bee's digestive epithelium, growing 

 there, sometimes changing form and wandering about in 

 the gut-cavity, or undergoing division so as to give rise 

 witliin the bee's stomach-cell to a number of spores which 

 may be set free in the intestine and passed out on to blossoms 

 where other bees may lick them up and thus become in 

 their turn infected. Bees harbouring large numbers of 

 these parasites may be unable to fly and show other symptoms 

 of disease, but it is exceedingly doubtful how far such 

 conditions may be due to the presence of the parasites. 

 Nosema homhycis, however, the microsporidian parasite of 

 the silkworm (Bombyx mori) was clearly shown by 

 L. Pasteur (1870) to be the organism causing the deadly 

 disease known as pebrine, which threatened sixty years ago 

 to destroy the silk industry in France. The caterpillars 

 may swallow the Nosema spores with the skin of the mul- 

 berry leaves on which they feed, and the minute, active 

 reproductive forms of the parasite may invade the ovaries 

 and enter the developing eggs in caterpillars that will grow 

 into female moths. Thus the embryo forming within the 

 egg-shell is parasitised, and the young larva, hatched in an 

 infected condition, is fated to be afflicted with the disease, 

 which may be passed on from one generation to another. 

 Such inherited transmission, relatively very rare among 

 animals, is of great interest to the student of biology. It 

 will be realised that disease incurred in this way is not, 

 strictly speaking, hereditary, because its appearance in the 



