Smerinthus Caterpillar Questions

[Liang]

This year I found an egg laid near a willow tree that turned out to be a sphinx of the Smerinthus genus. I have reared it on willow and it has pupated, but I am having a difficult time sexing it because I only have one pupa with no others to compare to. Here is the genital opening below on the fourth segment after the wings if anybody would like to help sex it? It weighs 3.2 grams if that helps.
 

While rearing the caterpillar, I also took careful note of its molts and I only recorded that it had four instars, though I had always thought they were supposed to have five instars. I took pictures of it every single day since first instar which can be seen on my website (www.lianginsects.com/general-insect-timeline/category/smerinthusophthalmica) and even saved every head capsule after each molt. Has anyone else reared Smerinthus caterpillars that only had fourth instars or is this an unusual occurrence? 

I am also unsure if it is Smerinthus ophthalmica or S. cerisyi because both are apparently in my region, the Bay Area in northern California. Ophthalmica was once synonymous with Cerisyi but was recently proven to be a distinct species. I have read that the mature Ophthalmica caterpillar has a blue horn while Cerisyi is usually pink, but I don't know if this is true or how reliable this identification method is. My caterpillar had a blue horn in its last instar, shown below, so I assume for now that it is S. ophthalmica?

Any insight or answer to any of these questions is appreciated.

[Paul K]

Female

[Liang]

Thanks, that's what I thought but wasn't sure since I didn't have a male to compare to. Anyone have any idea about the four instars or whether it is S.ophthalmica or cerisyi?

[eurytides]

Sometimes if a larva grows under very good conditions in terms of temperature and plant quality, it can skip an instar. It's not often mentioned in the literature, probably because many people don't keep careful track. I've raised polyphemus moths with 4 instars (i.e. 3rd directly to 5th basically). This year, I raised about a dozen promethea moths larvae, all of which spun cocoons after 4 instars.

[Liang]

Sometimes if a larva grows under very good conditions in terms of temperature and plant quality, it can skip an instar. It's not often mentioned in the literature, probably because many people don't keep careful track. I've raised polyphemus moths with 4 instars (i.e. 3rd directly to 5th basically). This year, I raised about a dozen promethea moths larvae, all of which spun cocoons after 4 instars.

Ah, I see, the larval instars are not very strict in number, rather it's however many it takes to reach the mature size. I know many caterpillars can have an extra sixth instar when reared under poor conditions or plant quality, but I didn't know it was possible to "skip" an instar under very ideal conditions. Often sixth instar caterpillars reared under poor conditions are larger than fifth instars under normal conditions, so I just hope my fourth instar caterpillar reared under ideal conditions isn't smaller than a normal fifth instar reared under average conditions, because that would just be ironic wouldn't it? Anyways, thanks for the information eurytides.

[eurytides]

My promethea cocoons look normal size. My polyphemus moths that only had 4 instars were about 10 cm in wingspan, which seems normal to me.

[Ascalaphus]

Very interesting. I am currently rearing L4 larvae of Smerinthus caecus and they are already so large that I expect there won't be a 5th larval instar. It's the first time I've ever noticed this in a Sphingid.

[Liang]

Yes, it is very interesting indeed. From what I've seen so far, Smerinthus ophthalmica naturally only have four instars, not five. The larval birth size for the species is disproportionately large for the adult size, almost as large as what you would imagine a second instar caterpillar would be when compared to moths of similar sizes. And although there are only four instars, they still manage to obtain large pupal sizes because they only shrink about 25-33% mass from peak larval weight to pupal weight, whereas from most other species I've seen it is about 40-60% mass loss. 

[Liang]

According to this source, tpittaway.tripod.com/china/s_cae.htm, Smerinthus caecus "may go through four or five instars, depending on the quality of the host". This apparently applies to several sphinx species of the Smerinthinae subfamily. I'm guessing four instars is probably the norm, but five is required when host plant quality is poor.