54 NOTKS ON A MALA.RU-CARRYING MOSQUITO 



insects body is like a lung. Air is received by openings on the 

 body called spiracles. Here, Fig. 17, is a photo, of one on the 

 pro-thorax, immediately behind the first pair of legs. It 's oval 

 and fringed with a row of hairs like eyelashes, for filtering out 

 dust. From the orifice large tubes proceed. These are built 

 with a spiral fibre running round the interior wall. The india- 

 rubber tube of a gas stove, with its spiral wire furnishes us with 

 an illustration of it. So these tracheal tubes are kept distended 

 for the passage of the air. They branch like a tree, gradually 

 growing smaller the further they extend. By this means air is 

 carried to all parts of the body. On each of the lower part of 

 the abdominal segments are a pair of these spiracles, but so 

 minute as easily to escape observation. This tracheal system 

 is the source of an immense amount of trouble, and annoyance 

 to the anatomist. Their tiny branches clasp every organ of the 

 body, and being so tough and elastic are very difficult to 

 separate without injuring them. 



In the interior of the body occupying the space between the 

 chest muscles and the stomach is a long transparent air sac, 

 filled with separate bubbles of air or gas, Fig. ly. The bubbles 

 vary in size from the three to two-hundredth part of an inch in 

 diameter. A transparent silk bag filled with india-rubber toy 

 baloons would resemble on a large scale the air bag of the 

 mosquito. The walls of the air vesicles consist of some oily 

 fluid, on rupturing the sac they float on water some time before 

 they disappear. On the back of the thorax, where it joins the 

 abdomen, are two club like organs projecting outwards. They 

 look like an aborted pair of hind wings. Here is a view of them 

 on the screen. The connection between them is simply a piece of 

 skin torn from across the beck. Scientists are not agreed as to 

 their function. They are called halteres or balancers, because if 

 one is cut oft' the insect cannot fly straight. They are there- 

 fore thought to be helpful in preserving a balance, just as the 

 pole of a tight-rope dancer enables him to adjust. By some 

 they are thought to be organs of hearing or of some special sense. 

 They are freely supplied with nerves, and are jointed at the base 

 so that they can be moved through the portion of the arc of a 

 circle. I have seen them sink down to the sides, and then rise 

 in a step by step motion until they have attained their altitude, 

 then sink and rise in the same way every six or eight seconds. 



We have in Fig. 21 a view of the stomach and other organs 

 of the abdomen. The air sac is attached to the upper part of 

 the gullet or iesophagus. The latter tube possesses strong 



