MALARIA EXPEDITION TO NIGERIA 237 
explanation that these insects should be carried on board for a considerable time after 
leaving the malarious district. The experience of the expedition afforded proof of 
the truth of this surmise. In making the tour of the ' creeks ' behind Bonny, on 
board the s.s. 'Sobo,' the first stopping place was Slave Trees, Bakana. This place 
is renowned in those parts for the number of its mosquitoes, which, as has been 
previously mentioned, are almost all of the genus Anopheles. The ship anchored 
about a quarter of a mile from the shore ; at night mosquitoes came on board in 
large numbers. In the cabins and saloons were great numbers of them. One night 
only was spent here. But for a week afterwards Anopheles could be observed in the 
darker corners of the saloons, and even ten days afterwards an occasional one could 
be caught, although the ship has been out to sea again in the meantime, in fact, had 
reached Opobo River. Probably in the smaller and closer quarters of the sailors 
they would have been found still later. 
In the report of the expedition to Sierra Leone, the following observations of 
the propagation of Anopheles occur : — 
' We also observed that while naturally-fed gnats invariably laid eggs after 
two or three days, those which had been bred from the larvae in captivity, and had 
then been isolated and fed in test tubes, never did so, although before being isolated 
they had long been in company with males. The inference is that fertilisation takes 
place only after the female has been fed. We noted also that, in a cage where many 
male and female gnats, both Culex and Anopheles, were kept together for weeks, eggs 
were never laid- — although the insects were fed, as described, on bananas, and the 
cage contained water for them to lay their eggs in. It seems, then, that a meal of 
blood is necessary before fertilisation. Lastly, we observed that previously fed and 
fertilised insects would lay a second batch of eggs after a second meal of blood, with- 
out a second fertilisation ; but never laid a second batch of eggs without a second 
meal of blood. That is, one fertilisation suffices for several batches of eggs, but one 
meal of blood for only one batch of eggs.' 
This is summed up in the following sentence — ' Although these gnats 
{Culicidae which feed on men) can live indefinitely on fruit, and perhaps juices of 
plants, the female requires a meal of blood, both for fertilisation and for the develop- 
ment of her ova. In other words, the insects need blood for the propagation of their species' 
Proof of the truth of these inferences is afforded by a series of experiments 
which we carried out, chiefly at Bonny. 
Experiment I — Four male Anopheles and five females hatched from pupae were 
placed together in a cage with a small pool of water and fresh banana : all died in four 
or five days, having laid no eggs ; on dissection, the ovaries were found undeveloped. 
Experiment II — Repeated the experiment with thirty to forty mosquitoes of 
each sex. It was noted that many, both males and females, fed on the banana 
immediately on introduction. All died in nine or ten days ; no ova laid. 
Q 
