THE ANATOMY OF THE ADULT MOSQUITO 105 



brought away attached to it, owing to the fact that the 

 fibrous base of the sacs, which connects them together 

 across the middle Hne, is divided into two bands, leaving 

 between them an opening through which passes the 

 oesophagus, a little behind the valve in which the latter 

 commences. To the naked eye they look like clusters of 

 minute air-bubbles, and when intact, their walls rival in 

 tenuity those of a soap-bubble. At the least touch they 

 collapse into a bundle of fibres amongst which a few 

 residual bubbles remain entangled. 



Instead of being as they have usually been figured, 

 barely larger than the salivary glands, they occupy, during 

 life, as much or more space than the digestive canal when 

 at its utmost distension. Moreover, the more gorged the 

 insect or the heavier it be with eggs, the larger will these 

 sacs be found, as together they occupy a large space and fill 

 out the entire ventral portion of the body cavity from the 

 front part of the thorax to the end of the fourth or fifth 

 abdominal segment. 



It is difficult to understand how any one could ascribe 

 to such a structure as this any active suctorial function, and 

 so describe it as " aspiratory," and there can be no doubt 

 that its function is identical with that of the air spaces 

 in the bones of birds, namely, to lighten the work of the 

 wings by increasing the bulk of the body ; and further, I 

 have now no hesitation in affirming that, as too, is the case 

 in birds, these sacs are mere extensions of the respiratory 

 passages. Into the base of the sacs may be traced large 

 tracheae and these split up and become continuous with 

 a brush of dichotomously dividing fibres of which the 

 base of each sac is composed. These fibres, like the intra- 

 glandular part of the salivary ducts, look much like those 

 of elastic tissue, and are undoubtedly composed of chitine. 

 ISIoreover, if we examine the collapsed " sac " under a 

 sufficiently high power, a number of minute air bubbles 

 will be found to remain, and close examination shows 

 clearly that these bubbles are not entangled between 

 the fibres, but contained in dilatations of their continuity. 

 In other words, these fibres are neither more nor less than 



