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Summer: Growth by molting

In the summer, Nephila spiders live on individual webs, capturing prey and growing by molting - shedding their "skin" - actually their external skeleton.

Juvenile Nephila are very small compared to mature females, and growing to maturity requires that they Molting spidersuccessfully pass through anywhere from 7 to 12 instars.  The variation comes in part from how much they get to eat, and in part from how long the growing season is.  I have studied both of these factors, and below are some of my own results.

The actual process of shedding an exoskeleton - called ecdysis - is very tricky for these spiders.  First they grow the new exoskeleton inside of the old one.  The new exoskeleton is chemically the same stuff as the old one, but it is not hardened yet and indeed is wrinkled like an egg carton to squeeze inside the old one, like having a large t-shirt on underneath a snug sweater. Some stretching will occur after the old skeleton is shed, but some of the new size must be there from the beginning. 

With the new exoskeleton complete, the spider dissolves the living tissue between the old and new skeletons, leaving only nerve connections to the sensory organs - touch-sensitive hairs, eyes, and chemical sensory organs (taste and smell) located on the outside of the body.  One day, the spider will hang from a thread on the orb web or under a leaf, disconnect those sensory organs, and pop the top on its cephalothorax (the body part with all the legs).  The body proper usually slides out easily, but then - as you see in this photo, the legs must follow.  It is not unusual for Nephila spiders, especially large females, to get stuck.  The chemical changes involved in hardening off the new exoskeleton start as the old skin is shed. Spiders not infrequently die as their long legs harden and stick before they can be completely withdrawn from the old exoskeleton.

To preserve flexibility of the joints, recently-molted spiders engage in stretching and flexing their legs in very standard sequences that are marvelous to watch.  An hour after the process starts, the spider is back on the web and, the next day, a new web is built and it is back in the business of eating.

Below are some questions I have asked about this process, the hypotheses I tested, and my results with links to published papers.

What determines how often a spider molts?
Some arthropods have internal clocks, and molt every time a certain number of days have passed - some of these even grow "backwards" to a smaller size when times are difficult. Other arthropods, including Nephila, must always increasing in size at the molt. I hypothesized that Nephila spiders have a "critical mass," and they molt every time they have gained enough weight to do so. 

The spiders gain weight by eatingData from Higgins & Ranking figure 1 insects.  If they capture fewer insects, then they should take longer to gain enough weight to molt.  I tested this prediction by collecting adult female spiders from a Texas population (Brazos Bend State Park) and letting them lay in the laboratory.  I reared their offspring to the fourth instar, then randomly assigned 10 spiders to one of five different diets, determined as a proportion of their initial body mass: 2.5%, 5%, 11.3%, 17% and 23% (this means they were fed between 1 fruit fly and 8 fruit flies every two days, no more and no less - a rather painstaking job!).

As the amount of food they ate increased, the rate at which the spiders gained weight also increased and the number of days they spent in the fourth instar declined, supporting my hypothesis (Higgins and Rankin 2001).  

mortality increases with dietOne surprising outcome of this experiment was that growing rapidly increased the risk of dying when molting:  The spiders that got the most to eat were much more likely to die. Similar death through over-eating has been observed in other spiders. 

It appears that spiders have no effective manner of detecting that they have eaten enough, and overeating is a mortal risk. It happens in the wild, also: one spider I tracked over several days in Panama had spun her web in what appeared to be a prime location beside a bee nest.  She always had large numbers of bees in her web and was eating when ever I checked on her, but she died the next time she attempted to molt.  An alternative explanation is that these high diets are somehow nutritionally unbalanced, but then why would spiders getting low and moderate amounts to eat do just fine even when it took them a month to get large enough to molt?

What determines how much a spider grows when it molts?
This is basically the same as the question "how much new exoskeleton fits inside the old one?"  Early in my carrier, my Higgins 1992 growth per moltprimary way of studying this question was to measure spiders before and after they molted.  Their abdomen expands as they gain weight (the exoskeleton is stretchy), but the size of the legs and cephalothorax are fixed, and only change when they molt.  When I plotted field data of size before and after the molt, the data showed a strong linear relationship - explaining about 90% of the variation.  

The arrow-heads in this figure from my 1992 publication show the average size of mature females in each of these three populations.  There are two arrow heads for Panama, because that population produces two generations each year and they mature at different sizes.

NclavipesInPanamaWhen ever I study new populations, I  always collect these data. Comparing the slopes of the lines - the slope being the growth at each molt - I have discovered that the amount of growth per molt is fixed (invariable) within a population, and varies little among populations. Even the giant N. pilipes of Papua New Guinea has the same amount of growth each time it molts: this parameter is constrained. 

These giant spiders get bigger by delaying maturing until they reach a larger size (more molts) but not by increasing how much they grow in each molt.  This is rather surprising, because it seems as though one of the easiest ways to get bigger would be to grow more in each molt – after all, the period of ecdysis is dangerous both because the spider may become stuck and because while molting the spider is defenseless against predators and parasites.

These observations raise two questions.  First is, why is growth per molt constrained?  I have no answer, but my hypothesis is that there are limits to how much new exoskeleton can be squeezed inside the old - perhaps limits to the folding.  I haven't yet figured a way to collect data to test this idea.

Second is a more approachable question.  What determines how many instars a spider goes through before molting to maturity?  I've worked a long time on this question, and it is really worth its own page, so if you are interested, go to the next page of Nephila life cycle by clicking on the spider button (below).
spider button How many instars does it take to reach maturity?
Publications
Here are some papers I've published describing development and testing these hypotheses.
  • 2010    L. Higgins and C. Goodnight.  Nephila clavipes females have accelerating dietary requirements. Journal of Arachnology 38: 150-152. 
  • 2002        L. Higgins.  Female gigantism in a New Guinea population of the spider Nephila maculata.  Oikos 99:377-385
  • 2001    L. Higgins and M.A. Rankin.  Mortality risk of high rate of weight gain in the spider Nephila clavipes. Functional Ecology 15:24-28
  • 1996    L. Higgins and M. A. Rankin.  Different pathways in arthropod post-embryonic development.  Evolution  50:573-582
  • 1995    L. Higgins.  Direct evidence for trade-offs between foraging and growth in a juvenile spider. Journal of Arachnology  23:37-43
  • 1993      L. Higgins.  Constraints and plasticity in the development of juvenile Nephila clavipes in Mexico.  Journal of Arachnology  21:107-119
  • 1992      L. Higgins.  Developmental plasticity and fecundity in the orb-weaving spider Nephila clavipes.  Journal of Arachnology, 20:94-106