OF THE HEAD OF THE IMAGO OF CHIRONOMUS. 269 



the surface which adjoins the enclosed cavity is be9et with very minute elevations of the 

 cuticle. A similar structure occurs iu more than one species of Ohironomus, and is 

 found, though less marked, in the female. The basal joints of the antenna? are closely 

 approximated in the male. 



The antennae of the female (fig. 12) scarcely reach half the length of those of the male. 

 They consist of seven joints only. The basal joint has the same shape as in the male 

 but is much smaller. Each of the next five joints is enlarged in the middle. The 

 terminal joint is elongate, but rnuch shorter, both absolutely and relatively, than that 

 of the male. The sensory hairs upon the five intermediate joints are comparatively few 

 and short, and the last joint only takes the form of a split tube. The basal joints are 

 much more widely separated than in the male. 



On the vertex, and between the posterior angles of the eye, are seen a pair of uiiuute 

 tegumentary processes (see also PI. XXIX. fig. 14 c), probably of little, if any, functional 

 significance. We find, however, that in the pupa they are connected with the brain by a 

 single median nerve. It may be of interest in this connection to recall a statement of 

 Dufour * that in Tipula oleracea, an insect belonging to a genus characterized by Meigen 

 and Macquart as devoid of ocelli, he found at the posterior border of each compound eye 

 a minute ocellary nerve terminated by a subglobular violet-coloured retina. He further 

 found behind the insertion of each antenna a minute subhemispherical tegumentary 

 prominence. Although failing to trace with certainty the connection between the nervous 

 and tegumentary structures so described, he hazards the conjecture that they are really 

 associated, and regards them as the functionless vestigiary representatives of the ocelli of 

 other Dipterous genera. 



The mouth-parts of the fly (fig. 14) are carried on a projecting process of the head 

 (rostrum), and consist of a labrum, tongue (lingua or hypopharynx), a pair of maxillary 

 palps, and a labium, subdivided into labellae. All these parts are imperfectly developed, 

 and almost or altogether functionless, except the maxillary palps. 



The rostrum corresponds to the fulcrum, or basal joint of the proboscis, in the Blow- 

 fly. Its upper surface is the part known as clypeus in Orthoptera and other insects. 

 A transverse suture divides this from the paired epicranial plates, which carry the eyes 

 and antennae. The rostrum ends below in a remarkable vaulted prominence, furnished 

 with sensory hairs — the epistome, to which the labrum is articulated. 



The maxillary palps are four-jointed, the basal joint being short and the other three 

 long. They retain in the imago the bent position in which they were developed within 

 the larval head. The labellse are devoid of pseudotrachese. 



At the base of the rostrum and on the dorsal surface are situated the superior orifices 

 (or) of a pair of large irregular, chitinous cavities, which extend through the head (figs. 15 

 & 10), each opening by a second minute slit-like orifice on its lower membranous surface 

 or gula. We are unable to give any explanation of these curious structures, but note the 

 existence of similar tunnelled cavities in the head of certain Culicidse, especially in 

 Anophilcs maculipennis. 



* " Eecherches anatomiques sur les Dipteres," Memoires preserves a l'lnstitut de France, torn. xi. 1851, p. 178. 



