142 BLOOD-SUCKING INSECTS 



not been ascertained ; indeed its presence can only be 

 assumed from the fact that the microscopic instrument 

 with which the puncture is made is quite incapable of 

 raising a blister and causing lasting irritation, unless 

 it is accompanied by some poison. The immediate 

 purpose of the irritation seems to be the promotion of 

 a flow of serum to the wound, whereby the blood is 

 prevented from coagulating. It has been surmised 

 that a similar effect is produced upon the juice of 

 vegetables when bitten by mosquitoes or midges. 



The common house-fly cannot bite, although it is 

 undoubtedly a carrier of disease by polluting human 

 food with its excrement ; but there is an insect easily 

 mistaken for it, which inflicts a painful stab, followed 

 in most cases by acute inflammation. This is Stomoxys 

 calcitrans, not uncommon in our gardens during the 

 summer, a near relation of the dreaded tsotse-fly. 

 Stomoxys bears a pretty close resemblance to a house- 

 fly, but it is somewhat smaller, and may be distinguished 

 from Musca domestica by the abdomen, or posterior 

 half of the body, being of a light grey marked by seven 

 black spots on its upper surface. I have had one eye 

 completely bunged up by the swelling from a bite on 

 the brow, which I attributed to Stomoxys, though I did 

 not catch it in flagrante delicto. People so attacked 

 sometimes attribute the mischief to a common house-fly 

 that has been feeding on carrion. 



Bed bugs, body lice and certain species of flea have 

 been ascertained to be carriers of disease among human 

 beings, but it is still doubtful whether any flying and 

 biting insect indigenous to this country injects noxious 

 bacilli in the act of sucking. Happily our islands do 



