106 INSECTS AND DISEASE 
to be the wrong season of the year for malaria, but nothing 
daunted, Ross commenced to investigate a closely allied 
organism, the proteosoma, found in sparrows. He had 
no difficulty in proving that round pigmented cells 
appeared in the stomachs of mosquitos that had 
fed on infected birds, just as he had formerly 
observed in malaria. He followed the development 
of the parasitic cells and found they gave rise, by 
internal sporulation to a number of fine rod-like spores 
which escaped into the tissues of the insect. And now 
came his crowning discovery. Dissecting an infected 
mosquito, he came upon two small glands connected by 
a duct with the proboscis. To his astonishment, he 
found the cells of these glands packed with enormous 
numbers of these parasitic rods. The inference that these 
rods were intended to pass with the secretion of the salivary 
or poison gland into the next bird bitten by the mosquito 
was obvious. He subjected it to the test of experiment, 
and found that birds whose blood was free of proteosoma, 
could be certainly infected by the bite of such mosquitos. 
That the malaria organism had a similar life-history appeared 
almost certain, and an Italian observer, Grassi, soon after 
confirmed Ross’s conclusions with regard to this parasite. 
The study of mosquitos was until recent years a much 
neglected province of entomology. But since Ross’s dis- 
covery, so much attention has been given to the subject, 
that they are now among the most completely known 
families of insects. We now understand the reason of 
Ross’s early failures in his investigation. Mosquitos may 
be divided into two groups—the Culex group, which com- 
prises the great majority of forms, both in individuals 
and species, and the Anopheles group, which are far less 
noticeable by the casual observer, and yet are exclusively 
concerned in the propagation of malaria. Without entering 
into technical details, it is easy to enable anyone to 
recognise the difference between members of these two 
groups, both in the larval and mature stages. Mosquitos 
of the Culex group have the larval form which is so familiar 
to us in our domestic water. These little wrigglers are, 
of course, air breathers, and ascend to the surface of the 
water to breathe through a conical projection, called the 
