Inhibition of pupation due to crowding in some tenebrionid beetles.

Pupation of mature larvae of several species of tenebrionid beetles is inhibited by crowding. The magnitude of the inhibition varies according to species. The primary factor causing this inhibition is the mechanical stimulation resulting from inter-larval contacts. Cannibalism of prepupae, pupae and teneral adults is discussed as the probable chief factor responsible for the evolution of the inhibitory effect of crowding. Of the many possible effects of population density on growth and development, those of greatest importance vary greatly depending on species and developmental stage (Allee, '34). Among insect larvae, the time required for completion of the larval stage may be shortened or lengthened by grouping, the effect sometimes being dependent on the degree of crowding. In a few cases, the principal stimulus and receptor mechanisms for these phenomena have been elucidated. For example, the acceleration of maturation in the desert locust, Schistocerca gregaria, is caused by a pheromone secreted by adult males of this species (Norris, '54; Loher, '60). By contrast, the acquisition of gregarious behavior in young Schistocerca and Locusta hoppers can be effected by purely mechanical stimulation (Ellis, '59), and mechanical stimulation is also a principal factor in the determination of winged forms in certain species of aphids (Johnson, '65; Lees, '67). This report was prompted by the observation that fully grown larvae of the large Central American tenebrionid beetle Zophobas rugipes Kirsch pupated rapidly in isolation, but failed to pupate under the crowded conditions of mass rearing although larval-larval molts continued to occur. Qualitatively similar behavior is displayed by larvae of several other tenebrionid species. To our knowledge, this is the first demonstration of an important effect of mechanical stimuJ. EXP. ZOOL., 276: 137-146. lation in the timing of pupal development. MATERIALS AND REARING The adults of the species studied were from the following sources: Tenebrio molitor L.: laboratory stock culture; Tenebrio obscurus F.: supplied by Mr. John Doyen; Eleodes armata LeC.: Panoche Creek, San Benito Co., California; Argoporis alutacea Csy.: Guaymas, Mexico; Amphidora littoralis Esch.: Antioch, California; Neatus tenebrioides Beauv.: Glastonbury, Connecticut. Larvae of Zophobas rugipes Kirsch were collected from a man-made cave near San Jose, Costa Rica, where they were living in fruit-bat guano. The larvae and adults of all species were maintained and reared satisfactorily on wheat bran and water.