The Pellagra-Fly. 



205 



Photo by] [E. K. Pearce. 



Eggs of Pellagra-Fly. 



The yellow eggs, in masses of several hundreds, 

 are laid on the leaves or stems of water-plants. 

 A portion of such a group is shown magnitied 

 four times. 



plentiful thousands of them will alight on a herd 



of cattle, and many of those bitten die in the 



course of only a few hours. In certain countries, 



such as the lands bordering the Danube, great 



destruction of cattle has followed the attacks of 



one species ; ^ in certain years the}- appear in 



such numbers in spring-time that they fill man 



and beast alike with terror. In these years 



thousands of cattle die from their attacks. They 



select those parts of the body that are not thickly 



clothed with hair, and are particularly attentive 



to the nostrils and ears. In the nostrils of beasts 



killed by them they are often found packed in 



layers. As the result of a bite, burning itching 



is experienced, followed by a painful, hard swelling, 



which may last for a. week or more. In cases 



where the bites are numerous and close together 



they produce an inflammatory fever, and in some 



bad cases cramps. We cannot go into the details 



of the disease as it affects the human subject, but we may state that the 



most obvious symptom is the thickening and reddening of patches of 



skin, which turn brown and scale off, and arc painfully sensitive to 



exposure to sunlight. These patches appear on the backs of the hands 



and wrists, the nape of the neck, and around the 



e^^es and nose. 



The fly, as in the case of malaria, yellow-fever, 

 and sleeping-sickness, is merely a carrier which in 

 its blood-sucking operations conveys the germs of 

 a low organism from an already afflicted animal 

 to one that is new to the neighbourhood, and 

 whose blood affords it the necessary aliment for 

 its development and increase. The species of the 

 genus simulium have not been as thoroughly 

 worked out yet as it is desirable they should be, 

 so that it is not at present possible to say whether 

 all the species arc blameworthy in this matter ; 

 but it is possible that they are, and Dr. Sambon 

 thinks that certain other small flies of blood-sucking 

 habit may be implicated. As in the cases of the 

 gnat (or mosquito), the midge, and the stout, it 

 is only the female flies that indulge in blood- 

 sucking. 



Taking one of our commonest species as a 

 type, let us glance at its life-history. The yellow 

 eggs are laid in masses of several hundreds on 



^ Siinuliiini columbaczense 



Photo by\ [E. K. Pearce. 



Grub of Pellagra-Fly. 



The hinder part of this remarkable grub is 

 furnished with a sucker and a circle of hooks, by 

 means of which it takes hold of the bottom of the 

 stream or of parts of the water-plants. On each 

 side of the niouth is a lobe ending in a fan-Iiko 

 arrangement of fine threads, with which it sweeps 

 the water to obtain food. .-Vbout four times the 

 natural size. 



