248 
SYDNEY J. HICKSON. 
DESCRIPTION OF PLATES XV, XVI, & XVII, 
Illustrating Dr. Hickson’s Memoir on “ The Eye and Optic 
Tract of Insects.” 
Reference Letters throughout. 
b. m. Basilar membrane. c. Cornea. ce. Cerebrum, co. Commissure. 
c. c. Crystalline cone. e. op. Epiopticon. n. Nucleus. Nf. Optic nerve- 
fibrils between the epiopticon and periopticon. n. c. Nerve-cell. n. c. s. 
Nerve-cell sheath, oe. (Esophagus. om. Ommateum. o. n. Optic nerve. 
op. Opticon. pc. Pseudocone. p. op. Periopticon. pg. v pg . 2 , pg. 3 . Pigment- 
cells. r. Retinulse. Rh. Rhabdom. rh. Rhabdomere. Tt. v Large tracheal 
trunk in the head. Tt. 2 . Smaller tracheal trunk. T. Tracheae with spirally- 
marked walls, t. v. Thin-walled tracheal vessels and vesicles. 
Fig. 1. — Diagrammatic vertical section through the head of Mu sea 
vomitoria, showing the relative positions of the cerebral and optic ganglia. 
ce. Cerebral ganglion, co. Commissure between the cerebral ganglia, o. n. 
Optic nerve, op. Opticon. e. op. Epiopticon. p. op. Periopticon. t. a. Ter- 
minal anastomosis, om. Ommateum. n. c. s. Nerve-cell sheath, oe. (Eso- 
phagus. 
Fig. 2. — Vertical section through the eye and optic tract of Musca 
vomitoria. In the greater part of the ommateum the pigment-cells have 
been omitted to prevent confusion, but the pigment-cells are drawn in the 
centre of the figure at pg. v pg. v pg. v In a similar manner the tracheal 
vesicles between the ommatidia are only inserted in two instances, t. v. Tt. x . 
The large tracheal trunk which sends the branches to the terminal auastomo- 
sis, t. a., to supply the tracheal vesicles of the ommateum. Tt. 2 . The smaller 
tracheal trunk which supplies branches to the optic tract behind the peri- 
opticon. 
Fig. 3. — Ommatidium of Musca, semi-diagrammatic, c. Cornea, pc. 
Pseudocone. pg. v Pigmented cells surrounding the pseudocone. pg- 2 . Ad- 
ditional pigment-cells. pg. 3 . Basal pigment-cells, n. pc. Nuclei of pseudo- 
cone. r. Retinulse. n. r. Nucleus of retinulse. Rh. Rhabdom. b. m. Basilar 
membrane. T. Trachea, tv. Tracheal vesicle, t. a. Terminal anastomosis 
sending fibres to the retinulse. 
Fig. 4. — Transverse section through the retinulse and rhabdom in their 
outer region, showing the six retinulse (r.) free from one another, and the 
rhabdom composed of six rhabdomeres fused together (rh.). The processes 
of the pigment-cells and their relation to the retinulse are shown at pg. 2 , and 
the relative size and position of the tracheal vesicle in this region is shown 
at t. v. 
