MONOCULUS, in natural history, a genus of insects of the order Aptera. Legs four to eight, formed for swim ming, and very long ; body covered with a crest, or shell, divided into segments ; antenna sometimes four, sometimes two, and sometimes without any; four feelers, in continual motion when swimming, the hind ones very small, and hook-shaped. There are about 50 species, separated into sections. A. With a single eye and crustaceous body. B. With a single eye, and bivalve shell ; antenna branched. C. With a single eye, and bivalve shell ; antenna simple. D. With a single eye, and bivalve shell ; antenna tufted at the tip. E. With a single eye, and univalve shell ; antenna two. F. Shell univalve ; two eyes placed beneath. G. Shell bivalve ; eyes two, placed on the back. The greater part of the Monoculi are very small water insects, requiring the assist ance of the microscope for the investi gation of their particular organs. To this there is, however, an exception in the M. polyphemus, which inhabits India.
This is distinguished by the title of the Molucco crab, or king crab, and grows sometimes to the length of four feet. In this species the eyes, instead of being approximated, as is required in the Lin nzan generic character, are extremely distant from each other, being situated towards the sides of the shell. " The whole structure of this animal is very remarkable, and particularly his eyes, which are between the fourth and last pair of claws on each side, reckon ing from his mouth, and excluding the small pair there placed, are inserted the rudiments of another pair, or a claw broken off on each side at the second joint or elbow ; on these extremities are the eyes like those of the horns of snails but under the covert of a thick • and opaque shell, nature in that place has wonderfully contrived a transparent lan tern, through which the light is convey ed." The great American king crab is very like this, but is specifically distinct.