254 J. M. DALZIEL. 



Relation of Crab-hole Mosquitos to Human Beings. 



With reference to the possible influence of crab-holes on the prevalence of mosquito- 

 borne diseases, the question will have to be decided on its merits in each diflerent 

 locality. In Tropical America two species, viz., Culex extricator and Deinocerites 

 . sp., have been found to "breed abundantly and exclusively in crab-holes," but no 

 evidence exists that either enters houses or shows any attraction towards human 

 beings, nor was blood ever found in the females of the Culex examined (Knab, 

 in " Science," June 1910, p. 869). Crab-holes in mangrove swamps of Brazil were 

 found to yield (along with Ceratopogoninae) Culex reticulatus, Lutz, C. corniger, 

 Theo.j and C. {Culicelsa) taeniorJiynchus, Wied. (Rev. App. Ent. i, p. 64), but there 

 is no mention of their habits in relation to man. In the Panama Canal Zone crab- 

 holes were dealt with in tens of thousands. 



According to the late Prof. Rubert Boyce, referring to Barbados, " In many of 

 the more low-lying swampy coasts crab-holes occur in enormous numbers in the 

 sandy soil, and in them are bred out vast numbers of mosquitoes. In fact they 

 constitute the chief nuisance in those houses which are situated near the sea." 

 (" Mosquito or Man," p. 96). Howard, Dyar and Knab (" The Mosquitoes of 

 North and Central America and the West Indies," p. 1) take exception to this and 

 indicate that crab-hole mosquitos, as observed by them, do not molest man, and 

 therefore their destruction need not be considered. It is apparent, however, that 

 the species under consideration by these observers difier in habit from those observed 

 by Professor Boyce, and this in turn is probably related to the important particular 

 of special adaptation, and the authors' conclusion must be read in the light of their 

 statement that the mosquitos that breed in crab-holes are " speciaHsed forms of 

 Deinocerites and related genera," and that there also occur " a number of species of 

 Culex peculiar to the crab-holes and even structurally these show more or less adapta- 

 tion to their habitat." {loc. cit., p. 149). 



In Lagos, of the species bred from larvae in crab-holes, all those which occur there 

 in appreciable numbers have been found also in a variety of other sources. No 

 species has yet been found here which shows a special adaptation structurally to this 

 form of habitat. 



Three of the species mentioned in Table I have not, in connection with the present 

 observations, been obtained as larvae from other sources, viz. , C. rima, U. hilineata 

 var. fraseri and M . inconspicuosus ; of these the two last named were each found 

 once only. It is unlikely therefore that the relationship in the case of these three 

 has any importance from the point of view of endemic disease. But the crab-holes 

 which produce abundance of possibly harmless species also furnish breeding-places 

 for no inconsiderable numbers of those credited with the spread of disease. 



The probable efiect of daily house-to-house inspection, with the main object of 

 preventing breeding of domestic mosquitos, in driving the Stegoyyvyia out of doors 

 has already been referred to. In circumstances such as those in Lagos, therefore, 

 the crab-hole in proximity to dwellings (a qualification which applies to practically 

 the whole of the inhabited area) deserves attention as an accessory domestic breeding- 

 place of the known vector of yellow fever. 



The connection of A^iojoheles costalis with crab-holes, I believe, on the other hand 

 to be more or less accidental and dependent on the flat contour and generally 



