278 MESSES. MIALL AND HAMMOND ON THE DEVELOPMENT 



transverse fold and showing the optic elements in course of formation; o.n, optic nerve 

 distributed to the convex surface of the eye, which surface afterwards becomes the concave 

 inner surface ; a.n', root of antennary nerve ; br, brain ; ces, oesophagus ; d.v, dorsal vessel. 

 x 50. The letters sec, sec , sec", indicate the levels of the transverse sections shown in 

 figures 18, 19, and 20 respectively. The lettering used in this figure will be repeated in 

 those following. 



Fig. 18. Early condition of the invaginations. Transverse section at the junction of the head and 

 thorax (sec, fig. 17). The longitudinal folds, /./, only, are cut through, ant', first trace of 

 the imaginal antenna, x 50. 



Fig. 19. Early condition of the invaginations. Transverse section through the broad part of the 

 transverse fold (sec', fig. 17). The cavity, t.f, of the fold appears as a thin cresceutic slit, 

 and the prolongations of the longitudinal folds are seen opening into its floor. The eyes, o, 

 are seen on the outer walls of the longitudinal folds. s', median space or sinus. x 50. 

 The great thickness of the larval cuticle is partly due to the oblique passage of the section 

 through it caused by the convergence of the body-walls toward the head. 



Fig. 20. Early condition of the invaginations. Transverse section through the narrow posterior part of 

 the transverse fold (sec" fig. 17). x 50. 



Fig. 21. Horizontal section through the head and thorax of a female larva, showing the formation of the 

 eyes and antenna within the longitudinal folds. The bulb of each antenna, bb, is seen in 

 two parts projecting into the cavity of the lateral fold from its inner wall. The parts lightly 

 shaded in this drawing and marked s, s', s represent body-cavity of the insect, the invaginations 

 being left white. The central one, *', is the median sinus referred to on p. 274 ; it contains 

 muscles connected with the labrum and the oesophagus, into it also project the frontal 

 ganglion and the termination of the dorsal vessel. The lateral ones contain the great 

 muscles of the mandibles, &c. The remaining letters as before. x 50. 



lTig. 22. Horizontal section through the head and thorax of a male larva. Condition tolerably 

 advanced but not ready for pupation. The antennary bulbs are as yet posterior (p. 271). 

 x 50. 



fig. 2.3. Earliest observed condition of the invaginations. Internal view as exposed by a median vertical 

 section. Only the longitudinal fold, /'./', is as yet present, the short extension of which, 

 /./", is referred to on p. 272. x 50. 



Plate XXX. 



Fig. 24. Female larva. Diagram showing the newly forming parts in a somewhat advanced condition 

 as laid open by a median vertical section. The inner wall, /./', of the longitudinal fold is 

 seen, and in its backward prolongation beneath the transverse fold, t.f, a large orifice is 

 represented as having been made exposing its cavity, /./, and the passage through it of the 

 newly forming antenna, the further course of which can be traced beneath the wall of the 

 fold. The pigmented facets of the eye are indicated on both the inner and outer wall of 

 the fold, more especially on the latter, which is seen through the opening, ant', imaginal 

 antenna ; b, its bulb ; f.ff, frontal ganglion ; d.v', dilated exti'emity of dorsal vessel ; s.g, 

 suboesophageal ganglion. The other letters as before. x 70. Note that the faceted inner 

 wall of the longitudinal fold is continuous with the antennary bulb. 



Fig. 25. Male larva. Same view as in the preceding figure. The bulb of the antenna is much larger 

 and projects towards the eye and towards the middle line of the head from the surface of 

 the longitudinal fold, instead of being sunk within it as in the female. The course of the 

 shaft has become arched upwards as described on p. 273. x 70. 



