Segmental sense-organs of arthropods

✍ Scribed by William Patten

• Publisher: John Wiley and Sons
• Year: 1889
• Tongue: English
• Weight: 152 KB
• Volume: 2
• Category: Article
• ISSN: 0362-2525
• DOI: 10.1002/jmor.1050020306

✦ Synopsis

THE cephalic lobes of Acilius are composed of three segments, each of which contains a segment of the brain, optic ganglion, and optic plate, the latter bearing two pairs of segmental sense-organs, or eyes. That these characters are common to all insects is very probable since the larvae of many Coleoptera, Neuroptera, and Lepidoptera agree with those of Acilius in having six pairs of ocelli and three optic ganglia, while in the adult the ganglion of the convex eye is generally composed of three lobes probably derived from three independent ganglia.

The segmental nature of the eyes is more clearly seen in the embryos of Scorpions, Spiders, and Limulus, where it can be shown that they are serially homologous, with one or more pairs of sense-organs on each segment of the thorax.

In Scorpions, whose cephalic lobes are composed of three segments similar to those of Acilius, the ganglionic invaginations of the first segment, which has no eyes, unite to form a transverse furrow that is soon converted into a closed sac, the walls of which are formed by the first segment of the optic plate, optic ganglion, and brain.

The second ganglionic invagination is at first like that of Acilius ; but the optic plate itself is soon infolded, forming the outer wall of the ganglionic sac, the inner wall being the optic ganglion. The two cavities finally unite in the median line, and the median eyes appear as a pair of thickenings on the inverted optic plates. A small group of upright cells soon appears on the