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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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6 responses

  1. Alice Pratt

    That was so interesting to read about. I am continually amazed at the intricate “web of life” surrounding us. We have an amazing amount of insects & plants in our yard. I discover new to me ones every year!

    August 27, 2015 at 8:06 am

  2. Jean P.

    Actually a question – in this photo – which is the old exoskeleton?

    August 27, 2015 at 10:15 am

    • Everything you see is the old exoskeleton — the carapace, or “lid” that opens up is at the top, attached by some fibers to the rest of the skin!

      August 27, 2015 at 1:23 pm

  3. This explanation of how the shedding actually works – the chemical and physical process that occurs as they get ready to slip out of the too-small exoskeleton – is so interesting! I’ve always wondered how this could happen.
    Adn… I hadn’t thought about this before, but for insects, is it only the ones whose development does NOT involve larval and pupal stages – in other words, only the insects that undergo “incomplete” metamorphosis, as nymphs – that have to shed exoskeletons as they grow? Or do some of the adults that emerge from pupa cases continue to grow, and shed their exoskeletons?

    August 27, 2015 at 2:35 pm

  4. Jean Haarrison

    Monarch butterfly caterpillars molt five times to become larger caterpillars then a pupa. The adult butterfly does not molt.

    August 28, 2015 at 4:51 pm

  5. judilindsey

    That is incredibly cool information! Wow! Thanks, Judi    

    August 30, 2015 at 8:58 am

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