Growth and Retrogression in Larvae of Trogoderma glabrum (Coleoptera: Dermestidae). 1. Characteristics under Feeding and Starvation Conditions1

Abstract

Under standard culturing conditions (basal rearing medium, 30°C, 65–70% RH) Trogoderma glabrum (Herbst) larvae displayed either 5 (males) or 6 (females) larvae stadia. No sex difference was detected in head-capsule widths of the successive instars, but the additional female stadium resulted in a pronounced sex difference in the size and weight of the prepupal instar. The larval growth period was from 26 to 30 days among the males, and from 32 to 36 days among the females; only a very small proportion of female larvae underwent a supernumerary 7th larval stadium and a delay of pupation. When isolated without nutrient medium, 6th-instar females failed to pupate and entered a period of retrogressive larval development. They molted periodically to progressively smaller larval dimensions (here termed “retromolts”). During 1 year of complete deprivation, the larvae retromolted from 5 to 8 times. Retromolts did not represent a simple reversal of larval development, in respect to weight and head-width; the weight and dimensional changes associated with a retromolt were much slighter than those accompanying normal larval growth, even though opposite in sign. Although all isolated larvae underwent the 1st retromolt within 2 weeks after isolation, subsequent retromolts occurred irregularly, and no temporal pattern could be discerned. The rate of weight loss tended to be highest among larvae that retromolted the most frequently, but retromolt frequency was not determined by the rate of weight loss. Headwidth following the retromolt was closely correlated with larval weight rather than with retromolt number. These characteristics were interpreted as suggesting that the physiological mechanisms controlling the molting cycle were relatively independent of those controlling metabolism and weight loss. The rate of oxygen uptake declined rapidly following larval isolation and stabilized at a level representing about 5% that of 6th-stage larvae on medium. During retrogressive development the larvae maintained a normal balance of body water, compensating for ecdysial water loss by absorption of atmospheric moisture. The adaptive significance of retrogressive development, in comparison with diapause, is discussed.