322 Mr. C. H. Goodman on the 



through openings called spkacles. In some cases in the 

 Myriopoda each sph-acle has its own separate tracheal system, 

 but this is seldom the case with the class we have to consider. 

 These tracheae consist of two thin membranes, between which is 

 coiled a spiral thread, thus afibrding the necessary flexibility 

 and firmness. The inner membrane is an invagination of the 

 outer cuticle, so that when a moult takes place a part at all 

 events of the tracheae is shed at the same time. Empty pupal 

 skins may occasionally be found floating on ponds with the 

 greater part of the trachea still visible, testifying to the deep 

 reaching nature of the metamorphosis. 



An idea of the distribution of the tracheas in the wings of 

 insects may easily be gained by observing the nervures, each of 

 ■which carries a branch, and which also assist in unfolding and 

 spreading those organs on emergence from the pupa state. la 

 the legs, too, they may sometimes be traced without much 

 preparation, and it is instructive to observe the way they are 

 enlarged in the neighbourhood of the auditory organ in the 

 tibia of crickets. The way the tracheae envelop the ganglia and 

 alimentary canal of the wasp are also well shown in the slides 

 exhibited. Closely connected with the trachete are certain 

 dilatations, found in flying insects, and called air-sacs. You 

 will notice that the structure is of a different character, and 

 that the spiral thread of the tracheae is discontinued. A con- 

 sequence of this is that they collapse when the air is withdrawn, 

 and it is owing to their necessary inflation before rising from the 

 ground that the apparently leisurely spread of the wings and 

 elytra of the stag-beetle is due before a start is effected. 



In Coretkra pkuiiicornis, the phantom larva, you will observe 

 two pairs of conspicuous spotted sacs, which. Prof. Kymer Jones 

 states, eventually burst and produce a tracheal system. In most 

 of the Diptera, two large air-sacs may be found occupying a 

 considerable part of the abdomen. 



Coming now to the stigmata or spiracles, great variety exists 

 in number, form, and position ; and it should be noted that this 

 variation often extends to the larva and imago of the same 

 insect. From one to ten pairs may be found, nine being a very 

 common allowance, while in aquatic larvae they are reduced to a 

 minimum. In shape they are either circular, oval, crescentic, 

 triangular, reniform, &c. As a rule, the thoracic spiracles are 

 the largest, although Dytiscus is a conspicuous exception. The 

 general position is down the sides, often in the soft integument 

 connecting each segment ; they are conspicuous in many of the 

 lepidopterous larvfe. Occasionally they are ventral, as in some 

 Hemiptera, but more often dorsal. In the common water- 

 boatman (Notonccta) a further admirable arrangement must be 

 noticed. The diagram shows a section of the abdomen, and the 



