Why do pupae turn?

[saye]

Apart from the lepidoptera that can be termed butterflies (generally Rhopalocera), and which are, at least, commonly held/hanged to a surface by the cremaster, moth pupae (generally Heterocera) are known to move/turn during the pupation period. Why?

A comparison to an avian egg might be imagined; eggs are turned so that the same temperature can be kept to its entirety. Obviously, not only are pupae not embryonic, nothing is sitting on them providing a constant temperature. Besides, pupae move on their own.

Turning and slightly moving has to have some necessity. In the wild, moving the slightest could be the difference between being detected by a predator or not. In this line, it is interesting to imagine that cocoon-building might have partly developed due to the protection conferred to a pupa that needs to turn around, rather then just the general banner of “protection”. As stated above, most butterfly pupae hang freely, without movement, and at times quite in the open. Their larvae have silk glands, but they spin no cocoons, even though that protection would be welcome.

Main question being then why do (moth) pupae move so much? How does this enter in the development of the adult?

Would appreciate some information, but imagined stuff as well.


PS: Although generally known to me, this has become grossly evident by the fuss made by saturniid pupae rustling inside their cocoons, which I’ve heard for days now.

[eurytides]

I can't remember where I read this, or if this applies to all Heterocera, but it has to do with orientation with respect to light/the sun.

[oehlkew]

Yes,
I think studies were done specifically with Antheraea polyphemus cocoons. If you pick up a number and handle them and then set them on a table, the pupae will move inside the cocoon to reorient themselves so light sensitive receptors are toward the light source.
This may help co-ordinate development with photo period. I think the reference may have been in Tuskes, Tuttle and Collins The Wild Silkmoths of North America, but I am not sure.

Bill Oehlke

[eurytides]

I think you are probably right Bill. It is one of my favourite books and I had a hunch that's where I read it, but I just wasn't sure. The book has so much first hand rearing information it's incredible. I think it's out of print now though.

[eurytides]

Actually, as it happens, I have a few A. polyphemus pupae right now. I picked up a few to check on them, and they sure move!

[saye]

The information pointed to sounds intuitive, but I wouldn't trust it to cover the entirety of the behaviour; or to put it another way, it might not be that simple. Though I am judging from ignorance since I can't check the reference and see how much it goes into the subject.

Thanks for the reference.

@ eurytides

What is the pupation period of your A. polyphemus individuals, if you've reared them before? I'm waiting on a few S. pyri (the ones I hear rustling, now less) and they are two days away from reaching a month, I'm not used to such a long period; though I imagine it the regular for saturniids.

[Adam Cotton]

As far as I know Saturnia pyri is univoltine, overwintering as a pupa, so they shouldn't hatch until next year.

Adam.

[saye]

Indeed, the species is univoltine. But the eggs provided were not F0; meaning, they were out of their usual time for occurrence. The first larva pupated almost a month ago, but in the wild the adults would have been on the wing until recently, perhaps some still are. So, by this, the pupae I have should eclose to be part of the univoltine cycle, that is: to mate, lay eggs, and have larvae grow until the end of Summer, when they will pupate and then settle to overwinter.

I really assumed this to be the case, that is, that this time-lag would allow adults to somewhat cheat on the regular cycle... am I completely off track?

[Adam Cotton]

I expect that they won't emerge until next year if the pupae don't experience a winter, but not being a Saturniid breeder please don't take my reply as definitive. I am sure that everyone here will be very interested to hear what happens.

Adam.

[saye]

I see the reasoning. But now I have trouble with causation: do the pupae enter diapause and remain in so until temperatures drop and then rise (like what general fruit trees go through, e.g. Prunus)? Or do they enter diapause because the temperature is low?

Off the hat, I have this to support the latter: Many overwinter two or more times, particularly if the springs are cold. Source.

Thanks Adam. I hope someone can contribute a bit to this.

[eurytides]

This is my first time rearing A. polyphemus. I found the eggs in the wild (brute force search) and the generation I got entered diapause. I'm not sure when they will hatch, but hopefully soon. I do not have personal experience with S. pyri, but Adam is right, it all depends if they go into diapause. Some species are obligate diapausers which means they NEED to experience a drop in temperature. The pupae need to "know" that the winter has passed before they start to develop into the adults. Then there are the species which are facultative diapausers which means whether they diapause or not is dictated by the environmental conditions during larval development. I'm not sure which group S. pyri belongs to. Bill Oehlke is the Saturniidae expert here - perhaps he can comment further.

[oehlkew]

From first hand experience I know that all that has been written about diapause and the need for cold temperatures is not uniformly true, even in the highly respected reference books.

Typically here on PEI in eastern Canada where polyphemus are normally single brooded, I will sometimes see a very small (less than 1%) of cocoons that I have harvested (sometimes several thousand harvested cocoons in a single year) in August-September even into October will yield moths that same year even though there would be no hope of a second brood successfully maturing before the cold weather sets in. Moths were emerging in August through early October indoors in very small numbers.

I have taken polyphemus adults at lights here as late as July 10 (female) which is typically too late for a second brood to mature from her offspring. Peak flight is normally early June to late June.

I grew up in New Jersey where there would be one, usually two, and sometimes a partial third brood of polyphemus, seemingly depending upon local conditions of temperature. If we had a warm early spring, and summer was hot, there would be at least a partial third brood. If there was a late, cold spring, followed by a cool summer, there might only be a single brood.

I rear lots of Hyalophora cecropia and H. Columbia Columbia here every year. Many years ago I reared a good number of cecropia indoors. One mature larva did not spin amongst the foliage but somehow mananaged to spin a cocoon beneath the baseboard hot water radiation cover. The cocoon remained there undetected throughout the fall and winter and emerged in early May. I assume its emergence was probably governed by an internal clock and probably by light photo period. Occasionally I have read from what seem to be quite reliable references that cecropia in very small percentages will attempt a second brood. I think this may happen in some of the southern states where larvae and cocoons may experience a similar photo period length in the increasing hours of daylight in the spring and decreasing hours in the fall. Perhaps if the larvae or pupae experience the same photo period length in the same year with temperatures sufficient for development to occur, that is exactly what happens, so long as temperatures allow for development.

This past spring I was surprised to see a male Hyalophora columbia Columbia flying around in my living room in very late May. It was much too cold out to be a wild male that somehow flew inside. We still had considerable snow on the ground at that time in shaded places. I assume that one of the larvae which had not pupated in a torn cocoon that I had harvested, escaped from a storage tub on the kitchen table and finished spinning under the table or again under baseboard radiation cover. For sure the cocoon/pupa never experienced any freezing temperature that winter. The coolest my house ever gets in the winter time is upper 50's F.

You will find it written in highly regarded texts that Columbia need to experience a hard freeze followed by warming in order to be able to break diapause.. When I first started rearing Columbia, I would keep the cocoons in sealed plastic tubs in fridge crisper in the fall and early winter and then put them outside for a week or so to make sure they experienced a hard freeze. Now I just keep my overwintering columbia cocoons, October to early May in sealed plastic tubs in the refrigerator crisper from late October until early May and I always get 100% eclosions of nice specimens that have never experienced a hard freeze. Crisper temperature never dropped below 38F. reproduction follows successfully.

Many many years ago I reared Saturnia pyri on pin cherry and I always found them to be single brooded. In some European countries they begin flying in very early spring while in other countries it seems they begin flight into late spring, always as a single brood. I am pretty sure that both times I reared them that I had at least one cocoon that spent two years in diapause even though all cocoons were stored in fridge in plastic tubs from October until May and all stock came from same parentage.

I think many of the single brooded species work diapause on some sort of internal clock, but there are probably temperature factors, photo period factors and possibly slight genetic differences that result in even siblings doing different things regarding diapause even under identical conditions.

You will just have to wait and see what happens. You might get a partial second brood, but I think that is much less likely than all pupae will overwinter, and you might even find that the odd one will spend a second winter in diapause. Please let us know what happens.

I also have first hand experience with H. columbia gloveri (western columbia) and H. euryalus and even Citheronia regalis spending more than one winter in diapause, and I have heard from very reliable sources that some African species spend multiple years in diapause especially during years of drought.

Bill Oehlke

PS My father once reported to me that some polyphemus cocoons he had sold to a zoo were left accidentally in cold storage from October of one year until spring TWO years later. The cocoons yielded healthy moths. That is not first hand experience, but I think one of these years when I have an abundance of cocoons I will intentionally keep some cocoons in cold storage for two consecutive inters without a warming period just to see what happens. I do not know at what temperature the zoo was storing the cocoons.

[saye]

I see that there is a lot going on with the variables that guide the pupation periods of these moths. Granted, when any one of them is considered alone, inconsistencies seem to creep up rapidly.

The species you mentioned are all from the Nearctic region, as is your locality, and this sounds a bit relevant to the need for pupae to experience a hard freeze. You see, in my region (in Portugal) there hardly is a hard freeze period! Regularly occurring S. pyri would never get anything but very sporadic 30ºF to 23ºF; though in general, 32ºF is the usual lowest. Snow is rare. I suppose hard freeze needs to be defined properly.

Of some importance though, is that this species is not abundant in the country, which restricts further any argument on the relevance of temperature since it is variable throughout the country.

As far as my eight individuals go, I initially opened one cocoon to take a look at the pupa. Much more recently, I opened another one due to intense ruckus I heard, to the point it almost sounded like the adult was scrambling inside. I opened it to see that nothing had happened; it might have been stimulated by its own prickly exuvia. I did however notice something that I’ve taken to mean that this individual, as well as the first, will not be forming adults this season. Both pupae (and I can presume the other six) have a clear slowly pumping artery, visible in the extension of the abdomen (dorsally centred), much as can be seen in a late instar larva. In this sense, it seems late for the process to continue.

The pupae (and larvae as they were) grew in a room that hardly dropped below 68ºF or so, and never went over 79ºF. They never experienced sunlight.
I would like to propose a very simple experiment. If I can get help in identifying a male and a female individual, I can store them in the fridge for seven days and expose them once more to summer temperature. Of course, if there are monitions to this sort of thing then please let me know, I have no intention of killing the individuals.

And, of course, thank you for the shared information.

[Adam Cotton]

It is easy to sex pupae of Lepidoptera with a hand lens. Here is how to tell the difference:



Also if you examine the antennae on the pupa the male antennae will be broader than those of the female in Saturniidae. In some species this can be less obvious than in most, but in many species the difference is really obvious.

Adam.

[oehlkew]

Saye wrote,
"The species you mentioned are all from the Nearctic region, as is your locality, and this sounds a bit relevant to the need for pupae to experience a hard freeze. You see, in my region (in Portugal) there hardly is a hard freeze period! Regularly occurring S. pyri would never get anything but very sporadic 30ºF to 23ºF; though in general, 32ºF is the usual lowest. Snow is rare. I suppose hard freeze needs to be defined properly."

Every winter on Prince Edward Island in eastern Canada we experience hard freezes with temperatures well below the freezing point annually often for a considerable number of days in winter. The Hyalophora columbia cocoons from this region definitely do not need a hard freeze to break diapause in the spring. I have overwintered cocoons of locally reared Hyalophora columbia for the entire winter in sealed plastic tubs in the refrigerator crisper where the temperature never gets below freezing but stay at roughly +3 to +4C or +37 to +40F, and I have reported moths hatching form columbia cocoons that probably did not experience a single date where the temperature dipped below +5F or +10C.

I cannot speak for species that I have not reared, but very definitely H. columbia columbia cocoons from PEI do not need to experience a hard freeze or even frost, even though in nature they do experience hard freezes throughout the winter. That comment is based on over 20 years of annual rearing experience of that species on PEI. I think they have an internal clock rather than straight photoperiod, because when stored in fridge crisper they do not experience a changing photoperiod.

Bill Oehlke