By Dr Michaela Betts and Dr Benjamin Kennedy
Tarantulas belong to the Theraphosidae family, and there are over 1,060 species spread over 116 genera, with over 250 being kept in the pet trade internationally. Males live for 5-7 years in most instances, whereas females can live for up to 30 years with the oldest tarantula recorded living to 43 years (and this one died due to parasitism rather than old age). Whilst frequently seen as part of zoological collections, keeping tarantulas as pets has been becoming more popular in recent years.
New World and Old World species
Tarantulas can be broadly separated into Old and New World species, with new world tarantula species being natively found in North, South, and Central America. These species tend to be more docile than their Old World counterparts, and their envenomation is comparable to a bee sting. These species have more ‘urticating’ hairs that they can flick into the environment or at a predator as a defence mechanism. Some people can have a severe response to these hairs and it is a big concern if they are flicking to eyes as they can be extremely fine and require specialist care (potentially surgery) to remove them. For this reason, the use of gloves and goggles during handling and rehousing to avoid these hairs is advisable.
Old World tarantula species originate from Asia, Africa, Australia, and Europe. Comparably these tarantulas are less hairy, may stridulate if provoked, and are anecdotally more aggressive. Bites from Old World tarantulas can cause intense pain, and in some species may cause spastic contractions of the muscle where bitten as well as local tissue necrosis.
Important anatomy & physiology
Tarantulas have four pairs of legs, which are attached to the prosoma (equivalent to a fused head and chest). They have one pair of pedipalps (these play a sensory and manipulation role in prey capture as well as in reproduction in males), and one pair of chelicerae (which contain the fangs and venom glands and are used for grasping and envenomating prey). Bulbous pedipalps and tibial spurs may be apparent in the males of some species.
The prosoma contains the oesophagus, sucking stomach, central nervous system and attachment muscles for controlling the limbs. The opisthosoma is separated from the prosoma by a narrow pedicle and is the equivalent of the abdomen. The book lungs (covered by two hardened abdominal plates), heart, hepatopancreas, excretory organs, and reproductive organs are located within the opisthosoma. On either side of the anus are two pairs of spinnerets that are responsible for the formation of multistrand silk filaments for web creation.
One could describe the spider circulatory system as “semi-closed”. The spider heart has an open venous circulatory system, meaning they do not have “true” veins for the transport of blood in the same manner as vertebrates, but they do have vessels that flow into the prosoma and then into the limbs. These are involved in maintaining hydrostatic pressure. The return of haemolymph to the heart is through a pressure gradient. Flexion of spider limbs (i.e. outward movement) is through muscular action and extension (i.e. inward movement) relies on haemolymph pressure.
Theraphosids have a rigid outer cuticle made up of the epicuticle, exocuticle, and endocuticle. The exocuticle is primarily made of rigid chitin to help form a protective outer layer. The epicuticle is important in restricting water loss and helps to also form the tough outer layer. Cuticle is best considered as spider skin rather than as a shell and has sensory receptors on the epicuticle. The rigid exoskeleton of spiders is also moulted for them to grow. It is a process that typically lasts for several hours, and venom ducts, book lungs, oesophagus and openings to the gonads are some of the major structures that shed.
Spiders can autotomise and regenerate legs and pedipalps and spinnerets. It is a voluntary defensive adaptation of the spider where it sacrifices the appendage to escape a perceived threat such as being predated. Legs will return to normal size within the preceding moults. Generally, the larger (and older) the spider, then the more moults are needed for the leg to fully regenerate.
The anorexic spider
Anorexia is a common complaint of captive tarantula species, particularly by novice keepers. However, in the absence of other clinical signs, “normal” behaviour should be ruled out and the set-up reviewed, particularly with inappropriate heat and humidity. The condition of the spider should be considered as a large overweight spider will be more likely to become anorexic. Condition in spiders is best determined by considering the ratio of size between the opisthosoma and prosoma.
Anorexia for weeks or months may be associated with the period preceding or following a moult. This occurs because the oesophagus is also moulted thus it isn’t a matter of the spider not wanting to eat but being unable to take food.
Male tarantulas who have emerged from their terminal moult may become anorexic in favour of trying to escape and find females with which to breed. It’s important to remember that adult male theraphosids only live for a limited time (typically 6-18 months) after their terminal ecdysis. Male tarantulas have enlarged palpal organs, and some species will also have hooked tibial spurs on their first pair of legs when emerging from their terminal moult. Conversely, female tarantulas may undergo a period of anorexia before producing an egg sac.
Common environmental conditions
As with all exotic species, appropriate husbandry is paramount to the health and well-being of the individual animal. Suboptimal environmental conditions can predispose tarantulas to medical and/or surgical conditions, some main offenders being: dehydration, trauma, and alopecia (loss of hair).
Signs that a tarantula is unwell may be visibly obvious or could be more subtle such as reluctance to move, abnormal posture, or anorexia. The latter is more difficult to determine given that some species will not eat for 6-12 months, and the Chilean Rose tarantula and Mexican Blonde tarantula have both been reported to sometimes go for prolonged periods without eating without a known cause.
Underlying reasons for lethargy and similar changes can also include infectious causes, parasites, and ageing. Parasiticide and pesticide toxicities should be considered carefully, especially for anti-flea products used on other animals in the house (such as cats and dogs). Spiders should not be handled with the same hands that have earlier stroked a mammal treated with an anti-parasite treatment. Treatment of intoxication is not often successful and so prevention is important in these cases.
Alopecia
Hair loss over the dorsocaudal parts of the opisthosoma may be seen in our New World tarantula species due to the urticating hairs being kicked off in response to a perceived threat or prolonged exposure to vibration. These hairs are replaced at the next moult but will not regrow before this, leaving the pale cuticle exposed. Recurrent defensive displays are indicative of environmental and/or handling stressors, so a full review of their set-up and their handling should be undertaken to identify what is triggering this defensive mechanism. This review should also include factors outside of their enclosure that may still have an impact, such as vibrations from speaker systems, the presence of predatory species in the vicinity, and banging on the glass.
This alopecia should not be confused with the normal alopecia of some tarantulas about to undergo a moult. In these cases, the cuticle tends to appear dark in colour and the opisthosoma has a full-looking appearance.
Dehydration
Most water is taken orally from the food source, but water should always be available in a shallow dish. Despite this, dehydration is a common presentation of our captive tarantulas. Theraphosids may also arrive to new keepers dehydrated if there have been postal transport delays (especially in warmer weather)
In some cases, the signs of dehydration might be more evident as deficits in movement or posture. This is due to the limb movement being dependent on hydrostatic pressure. In severe cases, dehydrated individuals may also have distortion or “shrinking” of their opisthosoma, the prognosis for these is poor.
Theraphosids who are responsive and able to move can be rehydrated by placing the cranial edge of their prosoma in a shallow water dish, taking great care not to submerge the underside of their opisthosoma and by extension their book lungs. Alternatively, some spiders may take water offered to them from a syringe with a soft cannula tip attached. These spiders will usually rehydrate within a few hours and may benefit from being kept in a warm humid set-up for 24 hours in the meantime, less if improving sooner.
Tarantulas are unable to access nor ingest oral fluids and will need to have injectable fluid therapy either into one of the joint membranes or into the heart directly. For these cases, a spider-savvy vet or nurse should be sought for these to be safely administered. The Veterinary Administration by these routes is not without risk.
Trauma
Trauma can occur for many reasons, from damage from bites from a feeder, to fall damage to limbs being damaged by decor. Death can occur rapidly from exsanguination (severe haemolymph loss) and so any ongoing drainage of haemolymph is an emergency. Some spiders will lose haemolymph gradually over several days and present as more lethargic perhaps with movement and postural deficits.
Substrates will often rapidly absorb haemolymph and so it can be difficult to identify. Placing the tarantula in question onto a paper towel substrate for 24-48 hours can facilitate monitoring for continued leakage of haemolymph.
Wounds at the base of limbs or over the upper part of the opisthosoma are likely to exsanguinate faster and so need to be identified as soon as possible. Post-mating injuries to the prosoma, though rare, have a poor prognosis and thus immediate veterinary care should be sought in these cases. Appropriate cessation of any bleeding needs to be carried out. Fluid therapy may also be required depending on the amount of haemolymph loss.
Keepers are also recommended to not interfere through direct pulling of the old cuticle during moulting as this is a common cause of fatal trauma. The underlying new cuticle is soft to enable body expansion and will not be as strong as the old cuticle being shed until up to 20 days after moulting. Consequently, any tension can result in a new cuticle tearing leading to catastrophic trauma. If a spider is trapped in a moult then manually extracting it should be the absolute last resort.
Housing
Tarantulas in the wild tend to be solitary animals and thus should be housed individually to avoid cannibalism. The exact design and size of an enclosure varies depending on the individual species, and keepers should be aware of these differences to provide a suitable environment.
Any holes for ventilation need to be smaller than the individual’s prosoma to prevent escape, and ideally smaller than the diameter of any legs to reduce the chances of any trapped legs. Wire mesh vivarium covers should similarly be avoided as spider toes can become trapped. Regular assessment of each spider and its environment is important as dehydration and/or anorexia can lead to a reduction in opisthosoma size and allow theraphosids to escape from previously appropriate enclosures.
Substrates should be tailored to the species, considering any need for burrowing behaviours. Horticultural vermiculite, coconut coir, peat, potting soil, oven-heated topsoil, and a combination of these are viable options for different tarantula species. Substrates should be free of pesticides and potential parasites, so topsoil and horticultural substrates should be avoided. Vermiculite dries out more quickly than other substrates so should be avoided for high humidity set-ups. It also does not provide good footing for breeding, nor maintain burrows well, so should be avoided for these spiders. Similarly, coconut coir does not easily retain its shape so is not preferred for burrowing species. Peat is particularly useful for burrowing and for high-humidity species but can be difficult to obtain ethically.
Ideal temperature gradients vary between species, with Chilean Rose preferring 18C-24C and Cobalt Blue preferring 26C-32C, so the thermal gradient should be adjusted for the species being kept. A maximum-minimum thermometer is recommended for close monitoring. Most tarantulas are photophobic (have an aversion to light), and vivaria should be kept out of direct sunlight and bright lights. External heat sources should not emit light so heat mats and ceramic heat emitters are viable options, though care should be taken to heat the side of the enclosure and not the bottom. Relative humidity also varies between species, with the Chilea Rose preferring 60% and the Cobalt Blue preferring 85-90%. Tarantulas should not be sprayed directly as it causes frustration and stress. Ideally, the side of the enclosure should be sprayed so it can evaporate into the air. All species should be offered water in a shallow dish, with water-soaked sponges being avoided due to the risk of bacterial and fungal contamination.
Arboreal species require taller enclosures, ideally with doors on the front and top of the set-up. They require some shallow substrate to help maintain humidity and plenty of opportunity to maintain a vertical resting position with appropriate climbing and hiding enrichment such as cork or pieces of wood. Terrestrial species may be obligate or opportunistic burrowers so should be provided with deep enough substrate to exhibit burrowing behaviour. Brachypelma vegans, for instance, have been known to burrow as deep as 1.2 metres, so the conventional advice of “6 inches” is often a gross underestimation of the required substrate. Half of a dark-coloured plant pot or plumbing plastic tubing are good examples of appropriate and easy-to-clean hides. The substrate mix should be able to support its weight to avoid the tarantula being buried (though some species have been known to bury themselves into the substrate).
Height of enclosure is an important consideration, especially for conventional species, as trauma and even death can be a result of inappropriately tall cage height. Similarly, the inclusion of sharp rocks or plants such as cacti should be avoided as enrichment items due to the risk of opisthosoma trauma.
Insecticides should never be used in enclosures. It may be better to avoid enclosures that have previously housed animals that have been treated with anti-parasitic medications to avoid any potential exposure.
Diet
“We are what we eat” is as true to spiders as it is to any other animal. Feeder invertebrates should be kept well and thus be of high nutritional value to any feeding animal (regardless of whether this is a tarantula, reptile, bird or mammal). Gut loading is often considered as being a replacement for feeding prey invertebrates, but we should consider that tarantulas are primarily carnivores and thus will be ill-adapted to absorb vegetative material (which is why we don’t see many plant-eating tarantulas). When feeder invertebrates are brought, they should be transferred to a separate enclosure and given a high-quality diet and kept at appropriate husbandry conditions so these feeders will put on condition and provide a good source of macro and micro vitamins. Overfeeding, particularly to promote more rapid growth in spiderlings, should be avoided as it predisposes to obesity and consequent trauma of the opisthosoma.
Feeding a variety of prey species may help to optimise amino acid proportions in the diet and provide more balanced nutrition. Invertebrates such as crickets, locusts, dubia roaches, mealworms, and fruit flies are all commonly reported as feeder insects. Wax worms should be used sparingly due to their high fat content. It is important to tailor the feeders to the natural behaviour of the tarantula in the wild, for example, an arboreal species may appropriately take flying prey. This factors both into the nutritional and enrichment value of prey. Mantids should never be offered as there is a risk of them overpowering and consuming a captive spider themselves.
Invertebrate cuticle contains calcium and zinc, though in different proportions to vertebrate bone, so some level of supplementation would be recommended though this is best achieved by giving vitamin supplementation (via a multicompetent supplement, not a sole calcium supplement) to prey populations over an extended period (i.e. not gut loaded). Excessive supplementation should be avoided.
Some larger species may accept whole, killed vertebrates and mammalian meat such as ox hearts. However, these will often take extended periods to be consumed and have a high fat and protein content compared to invertebrate species but may be appropriate for carrion feeders but should be used sparingly and not represent most of the diet.
Conclusion
Despite their increasing popularity and history of being kept in collections, there is still a lack of research and published literature on health conditions and the appropriate care of invertebrate species for both keepers and veterinary professionals. The Veterinary Invertebrate Society or an exotic vet with an active interest in tarantula medicine can be helpful sources of information on what can be done for tarantulas that are unwell.