Revista Chilena de Historia Natural 76 (3): 417-435, 2003
RESEARCH ARTICLE
Morphometry and sexual dimorphism of Elasmoderus wagenknechti
(Liebermann) (Orthoptera: Tristiridae) in two population outbreaks
The population ecology of rangeland insects has been little studied in Chile. There are
different species inhabiting the inter-valley areas of the transitional desert that, under certain environmental conditions, display
population outbreaks. One of them is Elasmoderus wagenknechti (Orthoptera: Tristiridae). It is an endemic species to
Chile. Its geographic range of distribution is from Taltal in the Second Region (25º22’S, 70º31’ W) to Salamanca in the Fourth Region
(31º46’ S, 70º58’ W). The results of morphometric analyses carried out on individuals captured from two population outbreaks are
reported and discussed in this work. The outbreaks took place in 1996 and 1999 in the Fourth Región (Coquimbo, Chile).
Elasmoderus wagenknechti has been described as a brachypterous species. In this work, the relation length of the
tegmen/length of the abdomen was 0.66 for the male and 0.51 for the female. To the metathoracic wing, this relation was 0.34 and 0.24
respectively. The morphometric variability of 23 characters examined in this work was low both in the male (CV: 3.04-31.46 %) and in
the female (CV: 3.68-32.00 %). Despite the thicker aspect of the female, the relation body width/total length was similar in both sexes
(0.27). Likewise, the level of hydration was close to 71% of fresh biomass in both the male and the female. In the same way, the
relation dry biomass/fresh biomass was also similar in both sexes (male: 0.28; female: 0.29). The morphometric comparisons from
different outbreaks showed 10 significant differences in males and eight significant differences in females. Besides body size (e.g.,
total length), the sexual dimorphism showed up in 12 morphometric comparisons. In all of them, the difference was favorable to the
female. The highest indices of morphometric variable ♀♀/ ♂♂ were dry weight (2.83), total length (1.56),
mesonotal width (1.55), and pronotal length (1.52); all of them related to body size. The condition index was higher in the female
(b<sub>1</sub>: 1.42) as compared to that of the male (b<sub>1</sub>: 1.25). Although this difference was not
statistically significant, a higher value would suggest a better fitness of the female. This fact might explain the ability of this species to
demographically outburst in the Chilean semidesert, once the right environmental conditions have been triggered
Key words:
insect morphometry,
rangeland insects, Orthoptera, population outbreaks
