250 J. M. DALZIEL. 



A single hole may furnisli eggs, larvae of all stages of growth, living pupae and 

 empty pupal cases, indicating uninterrupted use by female mosquitos for several 

 generations. The fact that in the great majority of cases the imagines captured were 

 unfed and apparently quite recently hatched, supported the assumption that the 

 insects were actually occupying the location of their birth. Other specimens are 

 probably females returning to lay eggs. On one occasion several holes along the base 

 of a line fence, which had been treated with tar emulsion a few days previously 

 without obhteration of the burrows, were found to harbour adult mosquitos. On 

 digging these out tarry water and tar-soaked soil were found but no larvae and no 

 crabs either dead or alive. Probably the amount used had been insufficient and the 

 creatures had succeeded in escaping through the soil from the end of the gallery or 

 by another bolt-hole ; the mosquitos were either dislodged individuals sheltering, 

 or gravid females preparing to lay eggs. 



Conditions which interfered wdth the work of eradication were mainly due to 

 the frequent occurrence of the holes along lines of fencing planted with shrubs and 

 trees separating one compound from another, under clumps of banana trees, at the 

 roots of ornamental plants, or at the edge of cement drains or masonry. 



Work on the above lines has been continued as circumstances demand up to the 

 present time. Along the Marina in the main business street of the town numerous 

 and very large crab-holes are found in which at times multitudes of mosquito larvae can 

 be demonstrated. Probably the ultimate disposal of these will be through extension 

 of the macadamised road surface to cover the whole space from the warehouses to the 

 concrete sea-wall. 



In Bathurst crab-holes have been found to be a proHfic source of Culicine mos- 

 quitos. As digging in of the holes failed to stop the nuisance, the crab making a 

 fresh outlet, the method adopted with success in some cases was to fill in with chloride 

 of lime and stamp down (Annual Med. and Sanit. Report for Gambia, 1914, p. 13). 



Species of Mosquitos found in Crab-holes. 



In the years 1914 to 1918 inclusive numerous samples of larvae found in crab- 

 holes were obtained and sent to the Medical Research Institute at Yaba.* There is 

 no continuous record for all the consecutive months of any single year, and during 

 the months of greatest rainfall, when more anti-mosquito work is done in the way of 

 oihng and ditching on a wider scale, samples from individual crab-holes were less 

 frequently sent, but the total number is sufficient to be of some value, and every 

 month in the calendar is represented. 



On several occasions also adult mosquitos were captured in the holes for comparison, 

 both with those found in neighbouring houses and with those ultimately hatched out 

 in the laboratory from larvae collected at the same time. 



No note has been made of other inhabitants of crab-holes such as Copepoda and 

 insect larvae other than Culicidae ; it is probable that species of Culicoides and 

 other so-called " sand-flies " wiU be found amongst those enjoying the same habitat. 



* I have to thank the Director, Dr. A. Connal, for undertaking their investigation and 

 Mrs. Connal for identifications of the numerous mosquitos obtained from these and from 

 various other sources. 



