Thursday, October 07, 2004

Manduca sexta pupae

This past weekend, the last of the Manduca sexta caterpillars from my recent batch of eggs wandered off of their tomato plants and began pupating. At about the same time, the first caterpillars that had wandered were finishing forming their pupal cases (it took them about a week; in case you're just tuning in, see my posts on 1st instars, 3rd instars, 5th instars, and wanderers for more information about the caterpillars' development.)

Side view of a newly formed Manduca sexta pupa. The shed fifth-instar cuticle is positioned above the pupa. The wings are visible in the, er, wing-shaped area covering the top half of the body.

What used to be a caterpillar has now molted and become a pupa. The pupa looks much more like an adult than a larva: you can see the developing wings, the pointed abdomen, the head and eyes, and even the tongue. The pupa is covered by a sclerotized (hardened) cuticle; it cannot feed, and has no mobile appendages, though the abdominal portion can rotate somewhat.

During pupation the insect's body undergoes a major reorganization. The nervous system grows drastically and restructures itself, new sensory structures are added (e.g. antennae), muscles are removed and rebuilt, adult locomotive features are constructed (e.g. wings), the digestive system is reorganized to adapt to a new diet, and the tracheal system is modified to provide for all these new tissues. All of this is done without feeding; the required energy comes solely from stores laid down during the larval stage.

The stem-like structure that comes out of the pupa's head is actually its developing tongue. The tongue grows out from the head, forms a loop at the base of the stem-like structure, and grows back towards the head. This long tongue enables the adult moths to feed on the nectar of deep flowers, such as Datura / angel's trumpet. The tongue will normally be coiled up inside the moth's head once it is an adult.

Top view of a Manduca sexta pupa. The head is at the top of the picture, and the wing attachment points are visible as small lumps on the thorax (the smooth surface just above the highly segmented abdomen).

I showed off the pupae during our recent campus open house, and the first question I was usually asked was, "How long before they turn into adults?" Unfortunately, there is no easy answer to that question.

As you've probably gathered based on M. sexta's three-week larval period, M. sexta can easily go through three or four full generations in a year. However, once winter arrives, M. sexta's host plants start to die out, and the cold weather hinders M. sexta's performance (they are ectotherms, after all). Thus, M. sexta would much rather wait out the winter and pretend it didn't exist.

To avoid the winter, M. sexta pupae can enter diapause, a state of delayed development. In their fifth instar the caterpillars measure the day length, and if the days are shorter than a given length, the caterpillars figure that winter is coming on and enter diapause immediately after pupating. During diapause the pupa may slowly develop, but the insect will not eclose until conditions have become more favorable (often measured by temperature). However, if the days are longer than the cutoff (I seem to remember 14 hours, but can't find a source to verify this), they will pupate normally and eclose into adult moths in only a few weeks.

Since I know neither the specific day-length cutoff of M. sexta, nor the number of hours of light these caterpillars were receiving, I'm not sure whether these pupae are going to diapause or not. So, I may get adult moths eclosing in a few weeks, or a few months; we'll just have to see.

It's interesting to note that not all insects have a pupal stage. Moths and butterflies (along with flies, beetles, and some other groups) are holometabolous insects, wherein the adult form is typically quite different from the larval form, and the two are separated by a pupal stage. Some insects' juvenile stages are very similar to their adult stages (e.g. grasshoppers and praying mantises), and these insects are said to exhibit hemimetabolous development; they do not form a pupa.

1 comment:

Radagast said...

Importing comments:

Temperance Acquistapace
I want to thank you for your site... I am growing Manduca for undergrad credit and i couldnt find even an assumption on the length of pupation... you could be my hero right now... THanks again.. Tempe
June 12, 2007, 2:18:36 PM PDT – Like – Reply


Oooh, I don't know. I've heard rumors that they might sense temperature variation and respond to that, but I don't know for sure (and haven't tried it). Given that they normally pupate underground, I'm not sure that they'd rely on light levels to end diapause.

Anyone else know?
May 23, 2007, 1:28:02 AM PDT – Like – Reply

Amy Payne
Question: Will exposing the pupae to a lengthened day (light timer set to 14 hours on, 10 off) decrease the diapause phase? My daughter's first grade class has been waiting patiently for two cocoons to eclose. (I think it will be a fun experiment anyway. We could use the second as a control except they pupated a month apart.)


Amy Payne
May 21, 2007, 5:55:30 PM PDT – Like – Reply

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October 20, 2004, 4:42:24 AM PDT – Like

You know me better than that ... if it was debris I would have cropped it out. However, your question is a good one because I only alluded to what that structure was in the figure legend.

Just like caterpillars shed their cuticle (molt) between each instar, they also shed their cuticle when they turn into pupa, and the "debris" is actually the cuticle the caterpillar shed to turn into a pupa. The head is on the right hand-side (the remnants of the head capsule are visible) and the horn/tail is at the left edge of the cuticle.
October 9, 2004, 2:42:30 PM PDT – Like – Reply

Semantic Compositions
Is the thing above the pupa something that was formerly part of the M. sexta, or was it just debris that happened to be in the pictures?
October 8, 2004, 11:53:44 PM PDT