|MadSci Network: General Biology|
>If a caterpillar consume only a certain color of milkweed from food >coloring, will its pupa change color? Hello, Thanks for sending in such an interesting question. I cannot answer your question directly, but I would infer (from the information I’ll relate below) that the answer is “probably not”. Some information: Insect pupae are usually immobile and are thus quite vulnerable to predation. In some insect species protection is afforded by pupation occurring within a cell or cocoon. For example, the larva of the Puss moth, Cerura, constructs a chamber made of wood fragments; these fragments are glued together to form a hard layer within which the pupa develops. Another species, the silk moth Antheraea can spin a cocoon of silk within which pupation occurs. However, the pupae of some butterflies, including the Monarch butterfly (Danaus plexippus), are not shielded in this way, and the coloration of the pupae may be its only protection. By blending in with its surroundings, it may be able to avoid detection by predators. The descriptions of Monarch pupae state that they are blue-green, later turning brown, (before the coloration of the maturing butterfly can be discerned through the transparent skin of the pupa). They also describe characteristic gold spots on the pupa. The only information I could find relating the diet of the Monarch caterpillar to the color of its pupae involved the role of certain chemicals from milkweed and the color of these spots. Monarch larvae feed solely on milkweeds of the family Asclepiadaceae. Some milkweeds are rich in chemicals known as cardenolides, and adult Monarchs can contain high levels of these cardenolides, which are known to make butterflies unpalatable to many predators (for example, many birds). Under normal circumstances, D. plexippus (the Monarch) is known to store cardenolides. However, caterpillars raised on a carotenoid-free diet produce pupae with silver metallic areas instead of the normal gold areas (and butterflies that do not contain cardenolides). Futher considering your question, it is worth asking how, and with what, you would dye the milkweed plant. In order to affect the color of the insect, I would think that the coloring would have to be associated with some part of the plant is actually taken into the insect and used, otherwise, it would probably just be excreted as waste. It is also worth looking more closely at why you want to know the answer. For example, are you interested in the way nutrients are taken up and utilized by the caterpillar? Are you interested in whether camouflage protects the pupae from predation? Something else? If you further define your question, it might be easier to either find out an answer, or design an experiment to answer the question yourself. On the topic of pupal camouflage, it is interesting to note that two other butterflies of the genus Danaus, (i.e. related to the Monarch), D. chrysippus and D. gilippus, can match the color of the background they are in, whereas Monarch pupae are always blue green. I hope this helps. I have listed some of the references I used to find out this information below. References: Urquhart, F.A. 1960 The Monarch Butterfly, University of Toronto Press. Rothschild, M., Gardiner, B., Mummery, R. 1978 Pupal Colouration and Diet. Antenna, London. 2, 15 Brower, L.P., 1984 Chemical Defense in Butterflies in Vane-Wright and Ackery, The Biology of Butterflies. Symposia of the Royal Entomological Society of London, 11. 109-134.
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