OVIPOSITION IN CERTAIN COLEOPTERA

Abstract

SUMMARY Methods for satisfactory oviposition studies of Tenebrio molitor L., Tribolium confusum Duv., Sitodrepa panicea L., Lasioderma serricorne F., and Dermestes vulpinus F. are described. Four types of oviposition cycle are recognized: Short‐lived species which lay all their eggs in a few days. Long‐lived species laying continuously over a long period. Rather short‐lived species laying batches of eggs at short intervals. Long‐lived species with two or more periods in which oviposition is more or less continuous, separated by a long period in which there is no oviposition. The normal oviposition cycles of Lasioderma, Sitodrepa, Tenebrio and Tribolium are described. Records of parthenogenesis in Coleoptera suggest that it is almost confined to the Curculionidae, and even there it is rare. Of the five species studied by the author, no virgin females laid any fertile eggs, and only those of Tribolium and Tenebrio laid any eggs at all. In many beetles which will copulate more than once, a single mating is sufficient to ensure that all or most of the eggs shall be fertile. In certain species, however, the oviposition rate falls if there are not repeated matings. This is shown to be the case in Tribolium, where a second mating, even in females still laying fertile eggs, has a marked stimulatory effect. There are usually both upper and lower limits of temperature for oviposition. In the experiments on Tenebrio and Tribolium it appeared that the upper limit was determined by the temperature which was soon lethal to the beetles; the lower limit lay between 14 and 16°C. In Tribolium it was shown that a sudden drop in temperature from 27 to 18°C. has a marked stimulatory effect on the oviposition rate of the beetles after they had been returned to 27°C. The conditions of atmospheric humidity and of food moisture‐content suitable for different species are very varied. Certain species, such as Dermestes, must drink water if they are to lay normally. Tribolium can live, and lay more than half of the normal number of eggs, in a flour and in an atmosphere approaching complete dryness. Tenebrio requires no water to drink, but even in 20 % relative humidity (at 27°C.) oviposition is very greatly reduced. Certain species, especially those of the short‐lived type, can lay eggs in empty glass tubes in the absence of any food or other oviposition site. Lasioderma. however, though short lived, requires an oviposition site; for this muslin will partially replace food. If it has first been fed on a suitable flour, Tribolium will lay a few eggs in pure starch, which is not a sufficient food for the development of the ovaries. It will not lay in empty tubes even if it has been fed. The changes in the appearance of the reproductive organs of Tribolium during the life of a female are described. After oviposition the degenerating egg follicles form the so‐called “corpus luteum”. This character is more easy to see when the ovary is empty, for example in a female in which lack of food has inhibited egg formation, than when egg production is continuing actively. It is shown that in Tribolium any external factor which interrupts oviposition will produce an ovarial cycle like that already described in the Scolytidae, in which family the interruption is normally due to hibernation or to the need for special food after laying a number of eggs.