104 



KNOWLEDGE 



[April 1, 1890. 



producinii; organ is coiiuected with the thoracic spiracles, 

 or breathing apertures, and the harsh and stridulating 

 character of the sound suggests what appears to be really 

 the ease, that it is caused by the vibrations of hard solid 

 bodies. A hemispherical canty intervenes between the 

 spiracle and the main tracheal trunk, and in this are 

 situated some hard chitinous processes, by the vibrations 

 of which it is believed that the sound is produced. 



Closely connected with the remarkable power of the 

 wings is the peculiar development of the thorax, which is 

 so characteristic of the Diptera. Eoughly we speak, with 

 reference to insects in general, of the second apparent 

 division of the body as the thorax ; but it by no means 

 follows that the part occupying that position and most dis- 

 tinctly visible when newed from above, is homologically 

 the same in all cases. The complete thorax is composed 

 of three segments called respectively pro-, meso-, and 

 meta-thorax ; but in the more highly specialised groups of 

 insects these three are not equally developed, and some- 

 times it is one, sometimes another of the three parts, the 

 development of which on the upper surface preponderates 

 over that of the rest. Thus in the Coleoptera (Beetles) 

 and Hemiptera (Bugs), what is frequently called the 

 thorax, on the dorsal view, is in reality only the prothorax ; 

 in the Hymenoptera (Bees, &c.) it consists mainly of both 

 meso- and metathorax, and in the Diptera almost entirely 

 of the mesothorax. This is the division to which the fore- 

 wings of all insects are attached ; and as in the Diptera 

 this is the only pair of wings that is developed as such, the 

 peculiar conformation of the thorax of a fly finds herein an 

 explanation. The chief thoracic muscles are not attached 

 to the wings themselves, but run from one part of the walls 

 of the thorax across to the opposite wall, so that the 

 greater part of the vibration of the wings is produced by 

 alternating changes in the shape of the thorax. 



At the base of each wing is a double membranous scale, 

 the " dtula" or " winglet " (Fig. 3) ; each of its divisions 



Fig. 3. — Right Winglet of Blieuotti.e (Oillijihorn). 



has a rounded and thickened outer edge, and the mem- 

 brane is extremely closely covered with minute hairs 

 similar to those on the wings, the larger part hanng in 

 addition some long flexible hau's arranged pretty regularly 

 but not closely in rows. The free edge of each is also very 

 closely fringed with delicate hairs. 'When the wings are 

 stretched out for flight, these scales form a continuation of 

 then- area as far as the centre of the hind margin of the 

 thorax. Hence the inner scale has, as shown in the 

 accompanying figure, a sloping edge where it fits under 

 the side of the triangular termination of the thorax. But 

 when the wings are closed, the outer scale is folded over 

 the inner along their line of junction, as well as imder the 

 wing itself, so that they lie one upon another like the 

 leaves of a book. It is impossible to say what is the sig- 

 nificance of these ahda- ; though connected with the wings, 

 they can hardly have much influence upon flight, and the 



peculiar arrangement of the double fold, together with the 

 remarkable profusion of hairs, seems to suggest some other 

 function. They reach their highest development in the 

 family Muxciild'. 



A httle distance beneath the larger fold of each ahila. 

 and entirely overarched and concealed by it, is an organ 

 (Fig. 4) which is highly characteristic of flies ; it consists 

 of a slender stalk carrying a globular 

 expansion at its outer end, and near 

 the point of attachment of the stalk 

 to the thorax are three minute sets of 

 rows of papill.e with hairs. These 

 stalked globes are called halteirs, 

 balancers, or poisers ; they are most 

 conspicuous in such flies as the daddy 

 long-legs, or crane-flies, and in the 

 bluebottle are reduced to extremely 

 small dimensions, so that they are 

 not likely to be noticed at all imless 

 carefully looked for. It is curious that 

 the development of the alula is always 

 in inverse proportion to that of the 

 halteies. Though so minute, their 

 structure is sufficiently elaborate to 

 suggest that they must be of con- 

 siderable importance in the economy 

 of the insect ; and many diflerent 

 fitnctions have been, more or less eon- 

 jecturally, assigned to them. Their 

 names, as above, indicate a notion 

 once current that they helped the 

 insect to maintain its eqiiilibrium 

 during flight ; they have, again, been 

 considered to be organs of hearing, or 

 to be in some way connected with 

 respiration. Situated as they are on 

 the metathorax, they appear to be the representatives of 

 hind wings. 



The legs of the bluebottle consist of the usual parts, and 

 there is nothing of special interest or huportance till we 

 come to the feet. All the tarsi consist of five joints each, 

 and are terminated by a pair of curved claws, under each 

 of which is placed a fleshy pad fringed with hairs, and be- 

 tween the pads is a straight sharp-pointed spine. In these 

 pads, or " pulvilli," as they are called, lies the secret of the 

 power flies possess of running over surfaces in any position, 

 often in defiance of gravity. A fly finds no more difficulty 

 in running up or down a vertical window-pane, or across a 

 ceiUng, than in walking over a perfectly horizontal surface. 

 This is not the case with all insects, many of which would 

 struggle in vain to mount a perpendicular glass surface ; 

 hence it cannot depend entirely upon the claws, for these 

 are developed in aU insects, and would therefore give all 

 equal facilities ; no doubt the claws are of some help when 

 the surface is at all irregular, as, for example, on a ceUing ; 

 but they can hardly be of much use in travelling over 

 glass. We therefore look to the puhilh for the explana- 

 tion. It was at one time supposed that their efficiency 

 depended upon atmospheric pressure, and that they acted 

 like suckers, the edges being closely applied to the surface, 

 and the centre part pulled up so as to create a vacuum 

 beneath. This explanation, however, seems to be nega- 

 tived partly by the absence of any mechanism to produce 

 such a vacuimi, and partly by the presence of great num- 

 bers of minute hairs on the imder sm-face, wliich could 

 hardly do otherwise than mterfere with such a close appli- 

 cation of the edges of the pad to the surface as is required 

 by the hypothesis. The mechanism which brings about 

 these curious results must rather be sought for in the hairs 



Fig. 4 — Balances 

 OF Bluebottle ( Cn/- 

 liphora). Showing 

 Basal Papilla; and 

 Nekte. 



