For Caterpillars: 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:

male

female

Caterpillars of Cephrenes augiades

For Butterflies and Moths: sometimes superficial characters of the insect can be used to determine the sex.

For example: some species are dimorphic, so the sexes have different wing patterns.

male

female

Eucyclodes fascinans moths

Some species just have bigger females

male

female

Anthela acuta moths

In moths, in general, the male moths have more and/or bigger pectinations on their antennae than females. So if you can get a male and female pair, you can tell which is which.

close-up of head, showing pectinations on the antennae of Abantiades atripalpis
(Photo: courtesy of Robin Sharp, Victoria)

With only a single specimen, comparison of the antennae with those in a collection of photos of the same species might help to see how much variation in the pectinations there is between individuals of that species.

Examining the sex organs at the tip of the abdomen through a microscope is the real way of determining the sex, but these organs are usually retracted internally, and are usually invisible without dissection.

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Australian butterflies

Australian Lepidoptera

Australian moths

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(written 26 November 2016, update 7 May 2023)