Molting

The exoskeleton of a spider is hard so is unable to expand in order to allow the spider to grow. To overcome this problem your spider will shed it's old skeleton, allowing a new and larger one to replace it. You may be wondering how the new shell can be larger than the old one if it has to fit inside the former. It is able to do so because it has an elastic consistency to it while it forms under the hard old skeleton. As the latter is shed the new skeleton pushes through and stretches before drying on contact with air initially, a layer of fluid separates the old and new skeletons, but this is ultimately absorbed into the new one and is replaced by a layer of air in order that the two skeletons do not become fused.

The actual molting normally commences when a split appears on the side of a carapace part of the cephalathorax.
This split continues horizontally along the abdomen and the old skeleton opens rather like a hinged lid.
The spider then pushes its new body out first and withdraws its legs from their former skeleton.
Once the spider has emerged it will rest for a length of time which can vary from just minutes with spiderlings to over 24 hours in the case of a large mature tarantula.
At such time you should not handle your pet or in any other way disturb it.
This could result in damage to its new and delicate skeleton.

In the wild, spiders are very vulnerable during their molting process and are often eaten by birds and reptiles. Even some of their own prey species may attack them at this time. Apart from this possibility, the actual molting process carries many inherent problems for a spider. If it is undernourished or dehydrated this may affect the chemical changes that take place as the new skeleton is formed, with many potential dangers. The skeleton may be too weak, or the spider may be too weak to shed its old skeleton. Baby tarantulas undergo a number of frequent molts, maybe up to four a year, and in general cope very well with these, which are finished quickly and without problems. Once a spider is mature it will normally molt once, sometimes twice a year until it reaches its maximum size. Then the molts may be less frequent and a required mainly to replace worn tissue or missing limbs. In the latter case the replacement limb often is smaller than other limbs but, after further months, eventually attains the same size.

Returning to the molting process, once this has been done your spider will start to flex its legs and fangs,this is to ensure they become very mobile. If the legs are not flexed enough the spider will walk with a more stilted action until its next molt. This may be because of malnutrition of for any other of many other possible reasons. Burrowing tarantulas often will spin a small web on the ground and then lie on their backs in order to molt, but this by no means always the case. While yet others may stay in an upright position and then lie on their sides when they actually push out of their old skeleton. Arboreal species normally will molt within the confines of their tube web. Prior to the molt you may notice that the rear of the spiders abdomen is shedding hairs and may turn to a dark blue-black colour - though you may hardly notice this change in a species of that colour. Also, an elderly male will start to lose abdominal hairs, but such hairs may be missing if they have been used as a defense mechanism, Some spiders will refuse food for weeks before they molt but others will except it almost up to molting time so you cannot categorically say when your pet is about to molt. Like wise, after a molt some spiders will not feed for days or even weeks, while others will do so.

Be sure to remove all food once the molt is underway and check that temperature and humidity levels are satisfactory so that these do not become a source of molt problems. Make a note of each date of each molt.

Once the male has attained maturity, which will take two to five years depending on the species, he will molt only a few times before dying. While he is a juvenile you sometimes may be able to sex him on account of his more swollen pedipalps, but this is by no means a totally reliable guide. Once mature he will, in numerous species, sport small claw-like hooks (tibial spurs) on his first pair of legs. The female on reaching maturity will molt many times and live a lot longer than the male, more than twice his age assuming she remains in good health.