How do caterpillars have sex?

Illustration by Slug Signorino

Cecil replies:

To cope with the rapid advance of human knowledge, we’ve been staffing up here at the Straight Dope, and one of the people I’ve been talking to is a fellow named George Angehr. George is an ornithologist with the Smithsonian Tropical Research Institute in Panama, which is all well and good, but I wanted to find out if he really had what it takes for membership on the Straight Dope Science Advisory Board. So I asked him to review the above question and got this response:

George, I thought, you’ve got the job.

Caterpillars, as anyone with the IQ of a plantain already knows, are sexually immature and thus can’t have sex (not that that stops some people I could name). However, George informs me that a critter called the common or evergreen bagworm (Thyridopteryx ephemeraeformis) is a sorta exception. It forms a cocoonlike bag with its head and legs protruding and dangles from a branch, looking, George says, like a small, ugly Christmas ornament. The male metamorphoses into your ordinary moth. But the female remains in the bag, having morphed into a “larviform” creature that still looks like a caterpillar, and thus ensconced awaits the ministrations of the male.

“When a male finds her, he mates with her by means of an elongate extensible copulatory apparatus that can be extended deep into the bag containing the female,” George writes. “OK, maybe the female isn’t technically a caterpillar, but she looks enough like one. And maybe the answer doesn’t feature two caterpillars going at it, but it does offer a moth boinking a caterpillarlike female with his enormous copulatory organ while she’s stuffed in a sack, which should be prurient enough to satisfy the Teeming Millions.”

Wait, George is just getting warmed up. “This also offers the opportunity to bring up certain gall midges, which in my opinion have the most repulsive life history in all of biology. The eggs of the parthenogenetic female larva develop inside her and hatch into other larvae that parasitize their own mother. They literally devour her from the inside, and when they emerge, she’s little more than an empty skin. The process is then repeated among each of the daughter larvae. As one of the references said, ‘greater love hath no woman.’ Maybe we should save this one for Mother’s Day.

“Which brings us to the entire topic of paedomorphosis (with its subcategories progenesis and neoteny), or reproduction by forms that retain juvenile features. Aquarists are probably familiar with the axolotl, a salamander that reproduces in larval form. One of the dominant features of human evolution is neoteny, in that we retain many features otherwise found in juvenile apes, in particular the disproportionate head.” George neglected to define progenesis (also known as paedogenesis), but from our previous work we know this means reproduction by juveniles. The classic case is the aphid, which as veteran readers of this column know is born pregnant.

George concludes: “It’s possible that some species of bagworm are like gall midges in that the larvae eat their way out of their mother’s body, although other researchers say she dies first and the young just hatch from the long-cold corpse. They’re dispersed by birds that eat the case containing the female and her eggs; the eggs resist digestion, so when the birds crap them out in a distant location, they’re still able to hatch.” I’m telling you, if you want to make a big hit socially, invite an entomologist (or George) to your next dinner party. The conversation will never flag.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.