The Detritus-Based Trophic System 425 



any time contains a high proportion of large, fourth-instar larvae. Aver- 

 age biomass is high, akhough the ratio of productivity to biomass is low. 

 The slowing of development that leads to multi-annual life cycles with 

 overlapping larval generations is an important contributor to the high 

 density and biomass of Diptera larvae in the tundra. 



In both cranefly species, respiration rate of larvae increases with 

 temperature over the entire range observed: 0.5° to 20°C. Qio values cal- 

 culated over this range are virtually identical: 2.34 for P. hannai and 2.35 

 for T. carinifrons, comparing fourth instar larvae. In contrast, the 

 growth responses to increasing temperature differ. The growth rate of P. 

 hannai increases with increasing temperature over the range of tempera- 

 tures observed in the field. Growth was fastest in mid-season, when tem- 

 peratures are highest. Thus, in P. hannai assimilation of energy must in- 

 crease by a factor greater than the increase in respiration rate as tempera- 

 ture increases; that is, the Q,o of assimilation is greater than 2.34. In con- 

 trast, Clement (1975) found a distinct growth optimum for T. carinifrons 

 at 4° to 5°C. Growth rate was reduced at temperatures above or below 

 this optimum. Assimilation of energy does not increase to compensate 

 for the increase in respiration at temperatures above 5°C. 



It follows from the differing growth responses to temperature that 

 Tipula carinifrons is an obligate arctic species that is unable to complete 

 development in warmer climates, while Pedicia hannai is a facultative 

 arctic resident that might also occur in warmer climates. In fact, P. han- 

 nai is known from a number of locations in northern Alaska, including 

 Anaktuvuk Pass in the Brooks Range, Umiat in the northern Foothills 

 (Weber 1950b), Meade River, and the Prudhoe Bay region (MacLean 

 1975b), areas varying widely in summer-season length and climate. T. 

 carinifrons occurs extensively in the Soviet far north (Chernov and Sav- 

 chenko 1965), apparently always in arctic coastal localities. In Alaska it 

 is known only from the coastal tundra at Barrow and Cape Thompson 

 (Watson et al. 1966), and from coastal tundra in the Yukon-Kuskokwim 

 River delta. 



Following the long period of larval development, the adult life span 

 is completed very quickly. Pupation begins in late June, and most adults 

 emerge quite synchronously around mid-July (MacLean and Pitelka 

 1971). In both P. hannai and T. carinifrons females have very small, 

 non-functional wings. In P. hannai the mouthparts, antennae, eyes, and 

 legs of females are poorly developed. Males of both species are winged, 

 but the wings are used only for feeble fluttering along the surface and 

 they are incapable of sustained flight. The morphological reduction of 

 females may lead to greater fertility (Byers 1969); the limited use of wings 

 by males indicates that wings would probably be of small advantage to 

 females anyway. Females of the third cranefly found in the coastal tun- 

 dra at Barrow, Prionocera gracilistyla, do retain wings and are capable 



