INSECTA. 



its lateral regions. They are always largest, as 

 in Neuroptera, in those species which are most 

 constantly abroad in the brightest light, and 

 are expanded over nearly the whole of the 

 epicranial region, as in Tabanus, Ckrysotoxum, 

 and Doros. But although the corneae of the 

 compound eyes are so largely developed, ihe 

 ocelli also are almost invariably present in this 

 order. They are generally three in number, 

 placed on the most vertical part of the epi- 

 cranium, immediately behind the proper corneae. 

 This is their situation in Musca, Helophilus, and 

 Stratiomys,and also in the gnat, Culex annulatus, 

 but we have not observed them in a neigh- 

 bouring genus, Pedicia. Professor Muller* 

 believes that the ocelli are designed chiefly for 

 observing near objects, and the fact of their 

 existing, as just stated, in many insects in 

 which the corneae of the proper eyes are ex- 

 ceedingly large, seems to favour this opinion. 

 Their presence, as in Hymenoptera, is most re- 

 markable in the males, as in the male Empidte, 

 in which, although the proper cornese cover 

 the whole surface of the head, yet there are 

 also three large ocelli situated in the trian- 

 gular space immediately behind the cornese, 

 and even elevated upon a pedicle. In Tabanus 

 there appears at first to be only a single ocellus, 

 situated in the median line between the corneae 

 at the anterior part of the head ; but on close 

 inspection it is found to be divided into two by 

 the longitudinal suture which passes through it, 

 so that the two ocelli from their close approxi- 

 mation appear but as one. 



In the organization of the mouth the same 

 parts exist in Diptera as in the preceding 

 orders, but modified in form to adapt them to 

 a different mode of use. Thus we have seen 

 that in Hymenoptera and Lepidoptera it was 

 simply necessary that the parts should be 

 elongated, to enable the insects to obtain the 

 liquid food already prepared for them ; but in 

 Diptera not merely was it necessary that this 

 should be the case, but also that their form 

 should be materially altered, to adapt them to 

 a mode of employment different from that of 

 analogous parts in other insects. Thus in 

 Tabanidtf, the labrum and mandibles are used 

 like lancets, to pierce the integuments of other 

 animals, before these parasitic blood-suckers 

 can obtain the living fluid they are in quest of; 

 while in other species, as in Eristalis jloreus, 

 (fg~ 379,) which subsists both on the pollen 

 and honey of flowers, the mandibles and 

 maxillae are employed to scrape off the pollen 

 from the anthers, before it is conveyed along 

 the tube formed by the united parts of the 

 mouth to the pharynx. In other Diptera, of 

 which the food is entirely fluid and easily ac- 

 cessible, as in the common house-flies, Muscidte, 

 all the parts of the mouth are soft and fleshy, 

 and simply adapted to form a sucking tube, 

 which in a state of rest is closely folded up in 

 a deep fissure, on the under surface of the 

 head, formed by the two sides of the clypeus. 



* Elements of Physiology, by J. Muller, M.D., 

 (translated by W. Baly, M.D.,part v. p. 1116. 



903 



Fig. 379. 



Mouth or proboscis of Eristalis jloreus. 



d, front beneath the clypeus ; e, labrum ; /, man- 

 dible ; g, maxilla and palpus ; i, labium ; i*, labium 

 dilated ; **i, inner surface of paraglossa ; ***, the 

 rows of hairs on the inner surface ; I, ligula j m f 

 cardo and submentum. 



On the other hand, in the (Estrida, which, as 

 we have seen in the Phryganidte, Bombycida, 

 and others that take no food in their perfect 

 state, all the parts of the mouth have entirely 

 disappeared. It is in Tabanidte that the oral or- 

 gans of Diptera are most perfectly developed, 

 and approach nearest to those of Hymenoptera, 

 and are easily distinguished ; while in the soft 

 fleshy proboscis of Muscida their identification 

 is a matter of considerable difficulty. Ac- 

 cording to Savigny* the proboscis of Diptera 

 is formed solely by the labium, or under-lip, 

 divided into its primary parts as in other in- 

 sects ; while Desvoidsyf on the contrary be- 

 lieves that it is not formed by the labium, but, 

 as in Lepidoptera, solely by the maxillae. Now 

 we have seen, that although in Lepidoptera 

 the maxillae alone form the tubular mouth or 

 proboscis, yet that in Hymenoptera the labium 

 is the part chiefly employed in gathering the 

 honey, which is conveyed to the cavity of the 

 mouth and pharynx only by the maxillae as- 

 sisting to form a tube of which the labium 

 constitutes the inferior portion. Analogy there- 

 fore would lead us to expect a somewhat si- 

 milar conformation of the mouth in Diptera, 

 and that since these insects have either to pierce 

 the coverings of other animals before they can 

 obtain their food, or to gather their nourish- 

 ment by employing the proboscis as a pre- 

 hensile organ, the maxillae may fairly be sup- 

 posed to enter into its formation, and accord- 

 ingly we find, on a careful examination, that 

 such is actually the case, and that the proboscis 

 is formed of the maxillae and labium united. 

 We have been led to this conclusion by a care- 



* Mem. sur les Anim. sans vertebres. 



t Essai sur les Myodaires, par le Docteur J. B,. 

 Robineau Desvoidsy, 4to, 1030, torn. x. Me- 

 moires de 1'Institut de France. 



