OF THE HEAD OF THE IMAGO OF CHIEONOMUS. 267 



This clear division of Metabolic Insects into two groups, with names, typical examples, 

 and definitions, was only possible when little was known of the subject. Dewitz * showed 

 that the imaginal disks of the Muscidae originate in deep invaginations of the epidermis, 

 and differ rather in degree than in kind from the shallow invaginations of such insects 

 as Corethra. They are, in fact, imaginal folds rather than imaginal disks, and it would 

 be convenient so to describe them in future f. It is the purpose of this paper to describe 

 a Dipterous type intermediate in certain respects between Corethra and Musca, and one 

 which throws not a little light upon the origin of the extremely complex invaginations 

 of the Muscidse. Other types might be named which are less specialized even than 

 Corethra. We have to get rid altogether of the hard-and-fast lines laid down by 

 Weismann when the subject was being explored for the first time, and endeavour to see 

 the origin and history of those complex imaginal folds which characterize the more 

 specialized Diptera. 



III. Description of the Larval Head of Chironomus. 



We must first give a short description of the head and its appendages in the larva of 

 Chironomus. The head (PI. XXVIII. fig. 7) is relatively small and of oval figure, widest 

 behind. The dorsal border of the occipital foramen projects backwards near the middle 

 line, while the ventral border is cut away so as to allow of flexion upon the thorax. The 

 middle of the dorsal surface of the head is occupied by a narrow plate — the clypeus, 

 whicb tapers to a point behind, and extends to the bases of the antennas in front. 

 External to the clypeus are lateral tracts, which carry the eyes. There are two pairs 

 of these eyes, which in Chironomus are apparently little more than pigment spots. 



The labrum (fig. 8) is bent round to the ventral side of the head, so that its free border 

 is directed backwards. This is armed with ten or more fine denticles. In the centre 

 of the ventral surface of the labrum is a squarish space, x, surrounded by a thickened 

 rim, and fringed by paired setae and hooklets. The whole surface of the labrum is 

 furnished with complicated organs of this kind, which are probably concerned with 

 the operations of spinning. 



The appendages of the larval head are the antennae, the mandibles, the maxillae, and 

 the labium. The antennae are short, and consist of about five joints, which diminish 

 rapidly in size towards the tip. Erom the free end of the first joint proceeds a long seta. 

 A little below the middle of the basal joint and on its inner side is a roundish space 

 enclosed by a chitinous rim %. This is probably sensory, and may perhaps be a simple 

 auditory organ. The details of the antennary joints vary according to the species 

 examined. 



* " Beitr. z. postembryonalen Gliedmassenbildung bei den Inseeten," Zeitschr. f. wiss. Zool. Bd. xxx. (Suppl.), 

 1878. 



t This is recognized in all recent works on the subject. Sec, for example, the very interesting discussion of 

 imaginal disks in Graber's 'Insekten,' or Mr. Lowne's new book on the Blow-fly (part i. 1890). One clerical error 

 in this last (footnote to page 77) requires to bo noted here. The insect described by Weismann in the memoir 

 quoted by Mr. Lowne was not Chironomus but Corethra. 



% Described and figured by Meinert, ' De Eucephale Myggelarver,' p. 436, pi. iii. fig. 78. 



iO* 



