432 S. F. MacLean, Jr. 



Life Cycle Length 

 Annual 2 Years 3 Years 



Weight 



of 

 Adults 



Climatic Severity 

 Growth Rate 



FIGURE 11-7. The resulting changes in 

 body size and life cycle length of Diptera 

 along a continuous gradient of increasing cli- 

 matic severity and decreasing growth rate. 



gradients (Mani 1962, 1968, Houston 1971). Similar changes, apparent- 

 ly, occur over latitudinal gradients (Hemmingsen and Jensen 1957). 



Body size and fecundity should continue to fall until the disadvan- 

 tage of reduced fecundity balances the disadvantage of increased mortal- 

 ity accompanying a lengthening of the life cycle. At this point the life cy- 

 cle will change discontinuously. The unit lengthening of the life cycle 

 should be associated with a discontinuous increase in adult size (Figure 

 1 1-7). Thus, we expect a saw-tooth pattern of size and life-cycle length in 

 species that are widely distributed along gradients of environmental se- 

 verity. Data are not available to test this prediction. 



ENERGETICS OF SOIL INVERTEBRATES 



Estimates of production, respiration, assimilation, and consump- 

 tion of energy were made for each of the major invertebrate groups 

 (Table 11-5). The source and reliability of the estimates varied with 

 amount of information available. Information was most complete for 

 the cranefly species T. carinifrons and P. hannai; production was esti- 

 mated from changes in the size distribution of larval populations in the 

 field, and respiration was estimated from laboratory measurements of 

 respiration as a function of temperature and size of larvae, extrapolated 

 to field temperature and size distribution (MacLean 1973, Clement 

 1975). A similar technique was used to estimate production of Enchy- 

 traeidae. Respiration of Nematoda, Enchytraeidae, Acari, and Collem- 

 bola was estimated using equations or parameters derived from the 

 literature. Calculation of energy budgets for these groups was then com- 

 pleted using bioenergetic ratios (e.g. production/respiration, assimila- 

 tion/consumption; Table 11-6) derived from published values (summar- 



