MICROBIOLOGY OF DISEASES OF INSECTS 639 



having a creamy appearance; the culture in the water at the bottom of the taiie is 

 so dense that the liquid becomes sirupy and has a strong alkaline reaction. 

 Dextrose, levulose, maltose and galactose are fermented; the inoculated medium 

 containing one of these sugars becomes acid at first, then alkaline. This alkalinity 

 is due to the formation of ammonia. B. acridiorum has lived over two years in 

 sealed tubes. 



Methods of Infection. — ^Natural. There are several natural 

 methods by which the epizootic is spread. Sick locusts or nymphs 

 leave their infectious liquid dejecta on the vegetation, the other locusts 

 eat the contaminated herbage, contract the disease and in turn infect 

 new plants thus continuing the cycle. With certain species of locusts, 

 the Schistocerca for example, another very important mode of contagion 

 exists: when one of their number becomes weak or where vegetation is 

 scarce both the nymphs and the adults eat one another. 



At the time of depositing the eggs, if the female or even the male 

 is diseased, the eggs will be forcibly soiled with the liquid of the diar- 

 rhoea and the bacillus will be conserved up to the time of hatching upon 

 the eggs or in the mucilaginous matter surrounding the eggs which the 

 locust has provided for their protection. 



A certain number of locusts among every swarm act as healthy 

 "carriers." Carriers among nymphs have never been found. 



The period of hfe of the insect affects its resistance. The afiult 

 locust is individually much more susceptible than the nymph. The 

 habits of life of each, however, have a great influence. The nymphs 

 are continually in contact with the vegetation and with each other as 

 they march in very dense columns; they are endowed with a voracious 

 appetite and undergo in the short period of their larval life, five molts, 

 which are the periods of their least resistance. The winged locusts, 

 to the contrary, passing a large part of their hfe in the air, are only 

 rarely pressed one against the other, except, for example, when the 

 weather is cold; they also eat much less than the nymph, thus the 

 epizootic will have a greater tendency to become generalized among the 

 bands of nymphs than among the swarms of adult locusts. 



The age of the nymph or locust influences its resistance; the young 

 jiymphs have a maximum resistance, but this decreases gradually, 

 reaching its minimum at the time of the last molt; the adult locust has 

 its minimum of resistance at the egg-laying period. 



The period of the molt is not a means of protection against this 

 type of disease, which is a generalized septicemia. 



