For Caterpillars there are two ways that may be used to determine the sex.
The developing sexual organs may be seen through the transparent skin of some caterpillar species, typically as a pair of yellow organs, one each side of the dark dorsal heart line:
For many species female caterpillars are larger than males of the same age.
For adult butterflies and moths there are basically nine ways you may be able to tell a female from a male.
showing feathery antennae.
showing threadlike antennae
The antennae of Lepidoptera appear to be the main organs of smell. Many moth species fly only at night, or have flightless females, and so use pheromones for sexual attraction. Usually it is the female which emits the pheromone. which the males smell with the antennae. So the antennae of male moths of many species has a more feathery appearance than that of the female.
2. SEX BRANDS
The male and female butterflies of many species in HESPERIIDAE differ in that the males have a long mark near the middle of each forewing.
The abdomen of the female has to carry eggs, so in many species the abdomen of the female is fatter, and the wings bigger than those of the male.
4. FLIGHTLESS FEMALES
In the families PSYCHIDAE and LYMANTRIIDAE and subfamily LITHOSIINAE: the female of some species is flightless and has no wings at all.
Some species are dimorphic, so the female has a different pattern and coloration on the wings from that of the male.
6. TERRITORIAL BEHAVIOUR
With some butterflies and moths, for example Papilio anactus, the behaviour of the sexes is different. The males continually circle around their territory, which is typically 100 metres across, whereas the females just fly across the land haphazardly. So if you see the butterfly or moth come past regularly say every 15 minutes, it is a male. If you see it once and never again, it was a female.
If you capture a moth or butterfly and keep it overnight, if it lays eggs it is definitely a female, and if it does not, then it could be a male.
8. ABDOMEN TIP
female Arthroschista hilaralis showing hair pencil on tip of abdomen
male Creatonotos gangis showing inflated coremata
The tip of the abdomen of some moths has glands for dispersing pheromones, and these are sometimes accompanied by tufts of hair or eversible organs. Usually it is the females that emit the pheromones, and have a tuft of hairs for that purpose, but in some species the males have eversible hairs or organs which emit male pheromones.
In some species, there is no obvious difference between the sexes, and one must resort to dissection of the tip of the abdomen to examine the genitalia under a microscope in order to work out the sex of a specimen. The sclerotised parts of the genitalia of the female and male adults evolve to become unique to each species, and so act as a lock and key, inhibiting sex between individuals of different but similar species.
(updated 6 November 2012, 25 December 2017)