268 MESSES. MIALL AND HAMMOND ON THE DEVELOPMENT 



The mandibles are long, pointed, and toothed. They do not, as is the case witli most 

 mandibulate insects, work in the same plane, but in planes at right angles to each other 

 (fig. 9), and when closed their toothed extremities meet at the centre of the serrated 

 plate described below as the submentum. The internal margin of eacb mandible is 

 fringed with long setje. 



The maxillre are stunted processes lying just behind the mandibles. The basal piece 

 is of irregular shape, sometimes produced internally into a setose prominence ; it bears 

 a stunted palp, which is flexible and setose. 



In the labium, which represents the second pair of maxilla?, all signs of the coalesc- 

 ence of paired organs have disappeared. A comb-like plate (sm, fig. 8) is conspicuous 

 on the ventral surface of the head, its denticles being directed forwards, so as to suggest 

 that they oppose the teeth of the mandibles. This comb-like plate one is at first sight 

 inclined to regard as the true labium, and this is probably Weismann's interpretation *. 

 But this plate is at least not the whole labium. On its dorsal surface, and whoUy 

 concealed by it, is a soft process (fig. 10) with frea anterior margin. It is beset with 

 minute chitinous plates and setae, and with papillae, some of which may perhaps be taste- 

 organs. Above this, again, lies the lingua, and between the lingua and the last-men- 

 tioned soft process is the opening of the salivary duct. It would seem that in the course 

 of development the labium, which was originally bifid t, becomes doubled upon itself. 

 The basal joint encloses the rest of the organ, and its fore edge becomes denticulate. 

 This basal joint, though usually named mentum, appears to correspond more closely with 

 the submeiitum of orthopterous insects. The soft process referred to above may be the 

 mentum. On each side of the submentum is a radiately striated plate or flap (y, fig. 8 & 

 fig. 13). 



The accompanying figures will explain many details which can hardly be made intelli- 

 gible by the most lengthy verbal description. 



IV. Description of the Head of the Imago o/" Chironomus. 



The head of the fly (PL XXIX. fig. 14) exhibits large crescentic compound eyes, 

 surrounding the bulbous basal joints of the antennae, in front of which are the rostrum 

 and mouth-organs. 



The antennae diff'er materially in the two sexes. In the male (PI. XXVIII. fig. 11) each 

 consists of twelve joints, the basal one being nearly spherical, and very large in comparison 

 with those which succeed. The next ten joints are short, and at first sight appear to be 

 cylindrical. These form, together with the elongate twelfth joint, the shaft of the 

 antenna. On closer examination it is found that the shaft is really a split tube, with a 

 double wall enclosing a central cavity (figs. 11 a & h). This striking peculiarity is a 

 modification of the more usual cylindrical form of antenna, and is attained during pupal 

 life by a doubling in of the wall of the organ upon itself throughout almost its whole 

 length, as will be seen further on. The exposed surface bears the long sensory hairs, while 



* " Entwicklung der Dipteren im Ei," Zoitsehr. f. wiss. Zool. xiii. 1863, p. 131, Taf. ix. fig. 45. 

 t Weismann, loc. cit. figs. 41, 42, & 43. 



