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Exoskeleton

Caterpillars Eating & Molting

The larval stage of a butterfly or moth is spent doing little but eating.  Only as a caterpillar will these insects have chewing mouthparts, and they waste no time in using them. As they eat, caterpillars increase in size and their skin (exoskeleton) becomes tighter and tighter, as it doesn’t grow larger.  The caterpillar grows a new, larger exoskeleton underneath the outer skin and then sheds, or molts, the old one. Most caterpillars molt five times.  At first, the new exoskeleton is very soft and not very protective, but it soon hardens. The shed exoskeleton is often eaten before the caterpillar ingests more plant food.

There are names for the caterpillar’s stage of development in between each molt, called “instars.”  When the caterpillar hatches from its egg, it is referred to as a “first instar” caterpillar.  After its first molt, the caterpillar is referred to as a “second instar,” and so on up until the exoskeleton is shed for the final time, revealing the chrysalis (if it’s a butterfly).

The Monarch in the photograph is a very new 4th instar instar caterpillar (see antennae which haven’t hardened).  It has shed three times.  Its third exoskeleton (which it has just shed) is on the milkweed leaf behind the caterpillar. To see a real-time video of a Monarch molting go to   https://www.youtube.com/watch?v=EbHyq3RwtxI.

(Thanks to Otis Brown for his keen eye in finding this Monarch caterpillar before it ate its just-molted skin.  Also to Ba Rea ((www.basrelief.org) for her instar confirmation.)

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Grasshoppers Molting

8-31-16 grasshopper molting 049A3466

Molting is the process by which insects and other arthropods grow. They have an external skeleton (exoskeleton) that supports and protects their body, unlike the internal skeleton of most other animals. Since the exoskeleton is hard and its outer layer is non-living, it cannot grow bigger by small increments as the human skeleton does. As an insect increases in size, it sheds the inelastic exoskeleton on the outside of its body, and replaces it with a larger, soft exoskeleton that has formed underneath the smaller, shed exoskeleton. Eventually this new exoskeleton hardens. This process is repeated several times during the life span of an insect (the exact number depends on the species).

Grasshoppers experience incomplete metamorphosis: they go through three stages in their life cycle – egg, nymph and adult. Nymphs are miniature versions of adult grasshoppers, except that they are usually light in color and do not possess functioning wings. Nymphs undergo five or six molts and with each molt their size increases and their wing pads progressively develop. Usually within a month nymphs molt for a final time, emerging as adults with fully developed wings. (Photo: shed grasshopper nymphal skin showing small, developing wing pads; inset – fully mature grasshopper)


Golden Tortoise Beetle Larvae Feeding

8-18-17 golden tortoise beetle larva 049A2063

When it comes to ingenuity, the Golden Tortoise Beetle (Charidotella sexpunctata) larva has all others beat! Instead of discarding its feces, it collects them and uses them as a means of chemical protection. Golden Tortoise Beetle larvae have a “fecal fork” on their last abdominal segment which they hold over their body. They also possess a muscular, telescopic anus which they can manipulate in such a manner as to deposit their feces onto their fecal fork. Bits of shed exoskeleton combined with days of feces accumulate on this fork and create an effective fecal shield. Golden Tortoise Beetle feces contain alkaloids from the plants that they’ve eaten (Bindweed and other plants in the family Convolvulaceae) and consequently the shield wards off predators. (Photo:  Golden Tortoise Beetle larva with fecal shield; inset – adult Golden Tortoise Beetle)

 


Spiders Molting Exoskeletons

shed spider skin 052Like other arthropods, spiders have a protective hard exoskeleton that is flexible enough for movement, but can’t expand like human skin. Thus, they have to shed, or molt, this exoskeleton periodically throughout their lives as they grow, and replace it with a new, larger exoskeleton. Molting occurs frequently when a spider is young, and some spiders may continue to molt throughout their life.

At the appropriate time, hormones tell the spider’s body to absorb some of the lower cuticle layer in the exoskeleton and begin secreting cuticle material to form the new exoskeleton. During the time that leads up to the molt (pre-molt period), a new, slightly larger, inner exoskeleton develops and is folded up under the existing exoskeleton. This new soft exoskeleton is separated from the existing one by a thin layer called the endocuticle. During the pre-molt period the spider secretes fluid that contains digestive enzymes between the new inner and old outer exoskeletons. This fluid digests the endocuticle that separates the two exoskeletons, making it easier for them to separate.

Once the endocuticle is completely digested the spider is ready to complete the molt. At this point a spider pumps hemolymph (spider blood) from its abdomen into its cephalothorax in order to split its carapace, or headpiece, open. The spider then slowly pulls itself out of the old exoskeleton through this opening.

Typically, the spider does most of its growing immediately after losing the old exoskeleton, while the new exoskeleton is highly flexible. The new exoskeleton is very soft, and until it hardens, the spider is particularly vulnerable to attack.

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Caterpillars Molting

6-25-14 caterpillar molting 044A caterpillar is the larval stage of a moth or butterfly. It is the only stage that has chewing mouthparts, and therefore a caterpillar spends most of its waking hours eating. This consumption of food results in massive growth, making its skin/exoskeleton very tight. When this happens, a hormone called ecdysone is produced, prompting the caterpillar to molt, losing its old exoskeleton (to left of caterpillar in photo) under which is a new and larger exoskeleton. After the molt, while the new exoskeleton is still soft, the caterpillar swallows a lot of air, which expands its body. Then, when the exoskeleton hardens, it lets the air out and has room for growth. Caterpillars molt four or five times as they grow. Each different caterpillar stage is called an instar. (Photo: Forest Tent Caterpillar)

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