Page:Tropical Diseases.djvu/743

XL] species of mosquito which have fed on the blood of a filaria-infested individual be examined immediately after feeding, the blood contained in the stomach of the insects will be found to harbour large numbers of living microfilariæ. If one of these mosquitoes be examined three or four hours after it has similarly fed, it will be found that the blood corpuscles have in great measure parted with their hæmoglobin, and that the blood plasma in consequence of this and of dehydration has become thickened, although not coagulated. If attention be directed to the microfilariæ in the thickened blood, it will be seen that many of them are actively engaged in endeavouring to escape from their sheaths. The diffused hæmoglobin has so thickened the blood plasma that it has become viscid, and holds, as it were, the sheath. This change in the viscosity of the blood seems to prompt the microfilariæ to endeavour to escape from their sheaths. They become restless, as if excited. Alternately retiring towards the tail end and then rushing forward to the head end of the sheath, the imprisoned parasite butts violently against the latter in frantic