It is a sexually dimorphic species with larger males who carry larger, curved horns.
It is a sexually dimorphic species - the female has pale yellow or greenish wings, the male is darker yellow with an orange patch on the forewing.
A recent study found evidence that, contrary to popular belief, size dimorphism between the sexes results from differential food conversion efficiency rather than differential amounts of food consumed.
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C. tubularis and its sister species, C. verrilli, are the only hermit crabs known to show sexual dimorphism in shell choice, with males using normal marine gastropod shells, while females use shells of gastropods in the family Vermetidae, which are attached to rocks or other hard substrates.
The sexes may be identical, as with Bismarck Kingfisher, but most species show some sexual dimorphism, ranging from a different bill colour as with Common Kingfisher to a completely different appearance.
Together with the Red-spectacled Amazon and the Yellow-lored Amazon, it is the only Amazon species in which adult males and females easily can be distinguished by external appearance (sexual dimorphism): Males have bright red feathers on their "shoulders" (alula), while females have green "shoulders".
It is similar to Agrius convolvuli, but there is slight sexual dimorphism (the forewing of the female is paler than that of the male), the lateral abdominal spots are buff (not pink) and the hindwing upperside pale bands are buff (not grey).
This pattern appears to be an example of Allen's rule and is seen in other New Zealand Pompilids but is less marked in E. insularis due to its greater extent of sexual dimorphism with the males always being more uniformly dark.
Sexual dimorphism refers to the phenotypic variations between male and female members of species.