Boy/Girl Caterpillars

[dawnm]

We know how to sex adult Monarchs. I will always cherish the moment when my youngest son, 4 years old at the time, was explaining how to tell a girl butterfly from a boy butterfly to his Kindergarten teacher (who had raised/released Monarchs for years in her classroom). Haven't had much time to research this on my own, but is there a way to tell the sex of a monarch caterpillar before it pupates, etc., ???

[Jim]

Unfortunately, the sex of monarch larvae can be determined only in dissection - so you probably don't want to know that badly However, for your information, male monarch caterpillars will have a testis located in the 6th abdominal segment, dorsal to the gut. If you have a last instar male caterpillar, the testis will appear as two bright red or pink sacs (though often they appear to be one sac).

Sexing monarch pupae is another matter and is pretty easy with a keen eye or a magnifying glass. Surrounding the cremaster (the structure from which the pupa hangs) are a series of rings, called abdominal sternites. Within the first ring (9th abdominal sternite) are several paired black dots next to the cremaster; turn the pupa so that you are looking at these dots. If the monarch is a female, the ring adjacent to the 9th sternite will have a line dissecting it; this line (indicated by the arrow on the diagram below) will be centered between the pairs of dots. Male monarch pupae do not have this line.



Diagram taken from Hughes, P. R., C. D. Radke, and J. A. A. Renwick. 1993. A Simple, Low-Input Method for Continuous Laboratory Rearing of the Monarch Butterfly (Lepidoptera: Danaidae) for Research. American Entomologist 39: 109-111.

[dawnm]

How interesting! Thanks for the insight. You're right though, I wouldn't want to dissect the caterpillar just to find that out, but now I can't wait to see if I can distinguish the pupas. Pretty cool!

[Sarah Dalton]

FYI, the biggest caterpillars almost always turn out to be female, I've found. We started noting the big ones and then checking the chrysalids after they formed.

[Pat]

Mine don't seem to have any correlation of gender to size. Need my bifocals to squint at features that small! Sorted out some still-green ones by what I think they are, M/F, and we'll see how I did in a few days.

Of the 8 that I had emerge today, the 2 that were obviously larger than the rest were males.