1978] 
Sivinski — Stick Insects 
401 
hours (62%). The difference in the mating durations between the size 
classes borders on significance (.10 > p > .05). 
Density and sex ratio are major components of the sexual 
environment and affect the extent of intrasexual competition 
(Parker 1974). Unlike the majority of walkingsticks, which are 
uncommon and widely dispersed (Key 1970; Craddock 1972), D. 
veliei and D. covilleae are locally abundant (as many as 22 
individuals in a 1.5 3 meter bush). Strongly male-biased adult sex 
ratios (up to 4:1) persist until late in the season, and are perhaps due 
to a combination of earlier male maturation and selective predation 
by birds during the early summer nesting period (Sivinski 1977). 
In 9 other phasmids known to exist consistently or occasionally at 
high densities (numerous enough to cause defoliation of trees or at 
least 5 individuals occupying a bush or refuge), the mean male 
length/female length ratio is .814 (Wattenwyl and Redtenbacher 
1908; Bedford 1975; Lea 1916; Gurney 1947; Wegner 1956; Stockard 
1908; Hetrick 1949; Severin 1911; Key 1970; Paine 1968; Wilkins 
and Breland 1951). Over % of the complete sample is more sexually 
dimorphic (p = .2201) (see Fig. 2). Kentromorphic phases, morpho- 
logically distinct forms existing at different densities, exist in several 
stick insects. As in locusts with density phases, dimorphism usually 
declines as populations reach their greatest concentrations (Key 
1957; Uvarov 1966). 
Individuals of Eurycantha spp. and Dryococelus australis are 
found in the closest spatial proximity of all known stick insects. 
Both congregate in tree hollows during the day (Gurney 1947). 
Aggregation sex ratios vary enormously and as many as 68 phas- 
mids have been found in a single cavity (Lea 1916; Bedford 1975). 
Length dimorphism is minimal (.908) and males have massive hind 
femora studded with spines of sufficient magnitude to have been 
used as fishhooks (Balfour 1915). 
Sexual dimorphism in some phasmids is exaggerated in compari- 
son to other orthopteroid taxa. Species with below average di- 
morphism engage in the longest matings recorded in the order 
( Acrophylla tessellata, male/ female = .493, duration 11 days, 
Korboot 1961; Anisomorpha buprestoides, .612, 3 weeks; Clark 
1974; N. sparaxes, .700, 1-79 days, Gangrade 1963; Timema 
calif ornica, .700, 5 weeks, Gustafson, 1966). Protracted couplings 
might contribute to selection for male diminution. Disruption of 
crypticity may be lessened and male maintenance cost kept to a 
