903 



INSECTA. 



fident. That they really form the anterior sur- 

 face of the canal or tube seems evident from 

 the distinctness with which coloured substances 

 are observed to pass along the tube when the 

 insect is taking food. 



There are various opinions with regard to the 

 manner in which the food ascends the tube to 

 the mouth. Some have imagined that it is 

 simply by capillary attraction, and others, the 

 chief of whom was Lamarck, that it is forced 

 along by successive undulations and contrac- 

 tions of the sides of the tube, occasioned by 

 the action of the transverse muscles. Kirby 

 and Spence* believe that these undulatory 

 motions, which certainly do exist to a consider- 

 able extent, are not sufficiently powerful to 

 carry along the food with the rapidity with 

 which it usually ascends, but that the lateral 

 canals, which, as we have just shewn, are the 

 proper tracheae of the organs, assist in pro- 

 ducing the phenomenon by occasioning a 

 vacuum in the mouth and tube which faci- 

 litates the conveyance of the food more rapidly 

 along it. That something of this kind does in 

 reality occur is proved by the following ob- 

 servation. We gave sugared water, coloured 

 with indigo, to two specimens of Pontia napi, 

 and on attentively examining the front of the 

 organ with a microscope while the insects were 

 busily employed in partaking of the fluid, ob- 

 served the particles of indigo disseminated in 

 it ascend along the tube, not in a gradual and 

 regular succession, as must have been the case 

 had the ascent of the fluid been occasioned 

 simply by capillary attraction, but pumped up, 

 as it were, sometimes in a full stream in quick 

 succession for one or two seconds, as if the 

 insect was then sipping a full draught, while 

 at others a few particles only ascended quickly, 

 followed by still fewer with a much slower 

 motion ; thus indicating distinct intervals be- 

 tween each draught or ascent of fluid. From 

 these circumstances we are led to offer the 

 following explanation of the manner in which 

 the food ascends the tube to the mouth. The 

 instant an insect alights upon a flower, it makes 

 a forcible expiratory effort, by which the air is 

 removed both from the tracheae that extend 

 through the proboscis, and from those with 

 which they are connected in the head and 

 body, some of which we shall hereafter see 

 are distributed over the oesophagus and ali- 

 mentary canal, and at the moment of applying 

 its proboscis to the food makes an inspiratory 

 effort, by which the tube is dilated, and the 

 food ascends it at the instant to supply the 

 vacuum produced, and is earned onward by 

 the same act to the mouth, and from thence 

 by the action of the muscles of the pharynx 

 into the oesophagus and stomach, without any 

 interruption of the function of respiration, the 

 constant ascent of the fluid into the mouth 

 being assisted by the action of the muscles of 

 the proboscis, which continue in action during 

 the whole time the insect is feeding. By this 

 combined agency of the acts of respiration and 



* Introduction, vol. iv. p. 470. 



the muscles of the proboscis, we are enabled to 

 understand the manner in which the humming- 

 bird sphinx extracts in an instant the honey 

 from a flower while hovering over it without 

 alighting, and which it certainly would be 

 unable to do so rapidly were the ascent of the 

 fluid dependent only upon the action of the 

 muscles of the organ. 



In Diptera there is the same irregularity in 

 the development of certain parts of the head as 

 in Neuroptera and Hymenoptera. The shape 

 of the head is usually that of a flattened hemi- 

 sphere, with its base or occipital region con- 

 cave, and approximated to the prothorax, as in 

 the common house-flies, Muscidtf, the blood- 

 suckers, Tabanida, and the gad-flies, (Estruhe. 

 But in others, as in the gnats, Culicida, 

 the long-legs, Tipulida, and the As&d*, 

 (fig. 349,) it is either convex at its occipital sui- 

 face or extended in the form of a short neck. 

 In the latter instances the occipital and epi- 

 cranial regions are large and distinct, and the 

 cornea are protuberant, and situated a little 

 anteriorly at the sides of the head, but do not 

 much encroach upon the epicranium. This is 

 not the case in the Tabanida, &c. in which they 

 occupy nearly the whole of the epicranial 

 region. But in most of the genera in which the 

 eyes are thus expanded, there is usually, as in 

 Neuroptera, some portion of the epicranial region 

 still existing in the form of a small triangular 

 space anterior to the inner margin of the corneae. 

 On this space the longitudinal portion of the 

 triangular suture is often distinctly marked, and 

 extends backwards between the corneas to the 

 occiput, as is well seen in Tabanus hovimts. 

 Anteriorly it extends as far as the middle line 

 behind the antennae, where it terminates, thus 

 distinctly indicating the proper boundary of the 

 clypeus posterior in this order. The whole 

 front of the head or face is formed of the two 

 clypei, which are so united together as to be 

 scarcely distinguished as originally separate 

 parts. They together form a broad and some- 

 what lozenge-shaped plate, at the upper 

 portion of which are situated the antennae, 

 and at the lower or anterior, which is notched, 

 the labrum, freely articulated with it, and 

 which is usually concealed beneath it. In 

 some genera, as in the Tabanida, the an- 

 tennae are inserted on each side of the middle 

 line, into little fossae close to the triangular 

 suture; while in others, as in Chrysotoxum and 

 Conops, the place of these fossae is occupied by 

 little elevations, upon which those organs are 

 seated, sometimes nearly close together, as in 

 Sargus. The face thus formed of the two 

 clypei is developed laterally on each side of 

 the corneae, and is gradually narrowed from 

 its upper part to its lower, where it is articulated 

 with the labrum. In Rhingia rostrata the 

 posterior clypeus is elongated, and forms the 

 long projecting front : it is deeply notched at 

 its interior margin, where, as also in Volucella, 

 is a very minute plate, apparently the ana- 

 logue of the clypeus anterior. The cornea, 

 as above stated, are usually the most conspi- 

 cuous parts of the head in Diptera, and form 



