Trevor T. Zachariah, DVM, MS
Taxonomy
Animals within the phylum Arthropoda are markedly numerous and diverse, inhabiting nearly every ecosystem niche that is known. However, only a relative handful is commonly kept in captivity.
Besides spiders, other members of the class Arachnida include the scorpions (order Scorpiones), whip scorpions or vinegaroons (order Uropygi), whip spiders (order Amblypygi), and sun or camel spiders (order Solifugae).
The subphylum Myriapoda includes the centipedes (class Chilopoda) and millipedes (class Diplopoda). These taxa include approximately 3,000 and 10,000 species, respectively.
Insects comprise the majority of the species in the subphylum Hexapoda. They are the largest group of animals, with nearly one million described species, and there may be many more as yet unknown.
Biology
Anatomy and Physiology
The body plan of all the arachnid groups listed above is similar, with a segmented body divided into regions. The cephalothorax (with carapace) and abdomen are connected without a pedicel in scorpions and uropygids; solifugids have a waist, and amblypygids have a pedicel. Appendage types and number are also similar. The chelicerae are reduced in all groups but the solifugids, where they form massive jaws for catching prey. Relative to body size, solifugid jaws may be the largest of any animal.13 Modification of pedipalps for predatory action occurs in the other groups, with the pincers of the scorpions the most dramatic. The abdomen of scorpions and uropygids is divided into the preabdomen and the postabdomen ("tail"). The last body segment of scorpions is known as the telson and contains a barb connected to paired venom glands. Scorpions also possess a pair of comb-like pectines on the ventrum, caudal to the last pair of legs, which sense chemical and tactile stimuli.
The internal anatomy of most of the arachnid groups is similar to that of spiders. Various modifications are found, however. One such example is the respiratory organs. Book lungs are found in scorpions (four pairs), and amblypygids and urpygids (two pairs), while solifugids have trachea similar to those of insects.13
The myriapod body plan is based on a long trunk of numerous nearly identical segments. Each segment is attached to legs, except for the head, first trunk segment, and the caudal telson. Whereas centipedes have only one pair of legs and trachea per segment, millipedes mostly have two pairs of each. Sensory structures include the eyes, antennae, and the paired anal legs attached to the last segment of centipedes. The tubular heart and digestive tract run the length of the body. Malpighian tubules are the excretory organs. The circulatory system is open.
Chilopods have a dorsoventrally flattened body, are extremely fast, and are all predatory. Attached to the first segment is a pair of forcipules containing venom glands and bearing fangs. The majority of diplopods are detritivorous, with a cylindrical shape and slow movement. The exoskeleton is strong due to significant calcification.13
Insects have a great variety of morphologies, but generalization of their anatomy is possible. The body consists of three segments: head, thorax, and abdomen. Three pairs of legs are attached to the thorax, as are sometimes wings. Sensory structures include a pair of antennae and compound eyes on the head, as well as a pair of cerci attached to the caudal abdomen. Respiration is served by a varying number of pairs of trachea. The circulatory system is open and consists of the heart and accessory hearts near the base of each appendage. The digestive system is relatively complex. Malpighian tubules are the excretory organs.
The external respiratory openings, or spiracles, of scorpions and insects can be closed, and their exoskeletons are waterproof, both of which serve to prevent evaporative water loss. Thus, both groups are able to withstand arid conditions. In contrast, the myriapods are all relegated to humid or moist environments due to their permeable exoskeletons and permanently open spiracles.
Restraint
Physical
Physical restraint of most terrestrial invertebrates is easiest and most practically performed using a clear glass or plastic vessel for containment. This is due to the small size of many species and the potential for injury to the handler, though it does impede access to the patient for close inspection or procedures.
Handling arachnids can be accomplished by gently prodding them onto an open palm. Scorpions may also be briefly picked up by gently grasping just cranial to the telson with fingers or forceps. Most millipedes will roll into a tight coil when handled, but if given time will often uncoil and begin to move. When moving, they should always be supported along the full length of their body.
Whenever handling an invertebrate, it is best to wear latex examination gloves. Handling of any chilopod or potently venomous scorpion is not recommended.
Chemical
Only anecdotal information about anesthesia of most invertebrates is available.3-5,9 Chamber anesthesia with an inhalant agent, such as isoflurane or sevoflurane, would likely be adequate for most species. However, information on the efficacy and safety of this procedure is not known. Instructions for constructing a chamber have been published,1,11 though less complex devices are adequate19.
Diagnostics
Anamnesis and Physical Examination
As with any patient, a thorough history should include signalment, husbandry conditions, feeding and excretion patterns, and information about past and present medical conditions. It should also include any information on the source of the animal, as well as the ecdysis cycle. Clients should be encouraged to keep records of all these phenomena. Physical examination should assess for abnormalities in physical condition, movement, and behavior.
Imaging
Standard survey radiography of invertebrates is often unrewarding. Elucidation of the gastrointestinal tract can be achieved by ingestion of radiopaque contrast material, though oral administration of such can be difficult in some species.3,6 Experimental use of micro-computed tomography has produced detailed images of internal structures of scorpions, allowing three-dimensional reconstructions of the vascular system.18 However, this imaging modality is not typically available on a commercial basis.
Sample Collection
Hemolymph of arachnids and myriapods can be collected in a manner similar to that of theraphosid spiders.16 Analysis for various cytological, biochemical, and blood gas parameters may be performed, but there is a dearth of reference intervals and interpretive information. Hemolymph collection from insects is often more difficult due to their typically smaller size.
Discharge from any orifice should be collected for cytology and culture and sensitivity. Fecal samples may be analyzed for microflora and evidence of parasites. Interpretation of culture results can be difficult, as the normal internal and external microbial flora of most species have not been fully discerned.
Therapy
Fluid therapy may be administered through a number of routes to invertebrates. In animals that are able to locomote, providing them with a readily accessible source of standing water from which to drink is often sufficient. For animals that are not mobile, they can be placed in a shallow dish of water, provided their orientation keeps the respiratory organs from being submerged. Water may also be administered directly to the oral opening with a syringe.
Parenteral fluid administration is also possible. Arachnid hemolymph likely has a relatively high osmolality, similar to that of theraphosids (> 350 mOsm/L).20 Among commercially available solutions, 0.9% saline (308 mOSm/L) is the closest match. For insects, 0.2–0.5% saline is recommended; however, this is not true for phasmids (i.e., stick insects), whose primary hemolymph ion is potassium.4 Injections can be made directly into the heart or body cavity; however, they must be made very slowly so as not to cause damage to the exoskeleton or internal structures.
Anecdotal accounts of topical and parenteral antimicrobial therapy appear in the literature,3,4 but no studies of efficacy or safety have been conducted.
Surgery
Amputation of an appendage is often necessary due to trauma or deformation. The most common amputations involve the legs, and are performed by inducing autotomy. The leg is firmly grasped with hemostats near its base (at the level of the femur near its articulation with the trochanter in arachnids). A sharp upward tug is then employed to excise the limb, and hemorrhage is often minimal. Unfortunately, since autotomy is typically a voluntary action, amputations should be performed without anesthesia to prevent excessive hemorrhage.
Autotomy is described in most of these invertebrate groups, except diplopods.8 Some insects and centipedes are capable of regenerating lost appendages. Most arachnids and millipedes are not.
Various methods have been used to restore integrity to the exoskeleton after a traumatic event. Tissue glue, other adhesives and sealants, and fine suture material have all been used with varying success. The outcome often depends on the extent of the injuries.
Diseases and Conditions
Trauma
Traumatic lesions most frequently occur when an animal is dropped or falls from a significant distance. Any injury that destroys the integrity of the exoskeleton causes hemorrhage and has the potential to lead to death from hypovolemia. Thus, it should be treated as a medical emergency. The common sites of injury are the appendages and the abdomen.
Dehydration
Proper arachnid husbandry includes allowing access to a pool of standing water from which to drink. Without this, dehydration can occur and can be difficult to detect due to the rigid nature of the exoskeleton. However, clinical signs become apparent when it is severe enough to cause hypovolemia. These include paresis and contraction of appendages.
In other invertebrate species, dehydration can be even more difficult to detect. Often, vague clinical signs of illness or acute death occur.
Dysecdysis
Improper or incomplete molt is a less common problem for insects than for myriapods and arachnids. It can lead to traumatic injury, deformation, and even death. Likely contributing causes are similar to those for spiders, and include age, poor physical condition, disease, injury, and low enclosure humidity.14 Additional causes in millipedes may be overcrowding3 and inadequate nutrition (i.e., hypocalcemia). If an animal has not completed a molt within 24 hours, intervention is likely needed. Application of glycerin to soften the old exoskeleton or soap solution for lubrication may be used in combination with gentle traction. The old exoskeleton may actually need to be cut off of the animal. Whatever the strategy, great care must be taken to avoid injury to the fragile new exoskeleton.
Parasitism
It is unclear whether mite infestations should be considered parasitic infections in captive invertebrates. This is often the case in arachnids. However, in millipedes such mites are considered commensal organisms.17 The author has also witnessed this phenomenon in captive hissing cockroaches (Gromphadorhina portentosa). Even if not parasitic, mites are typically considered to be aesthetically unappealing. To eradicate them, close attention to cleanliness of enclosures, along with repeated removal of the mites from the animal with a damp swab or fine artist's brush, is needed. The use of predatory mites (Hypoaspis sp.) has also been described.2
A number of internal and external parasites affect insects. One of the most famous examples is the mite Varroa jacobsoni that affects the honeybee (Apis mellifera).
Infections
Numerous bacterial and fungal species serve as opportunistic pathogens of invertebrates. Bacillus spp. in particular seem to have a high level of pathogenicity for these animals. There are also a number of known viral and bacterial pathogens.
Human Health Hazards
Punctures and Bites
Wounds from spines, barbs, and mouthparts are often painful and may require first aid. The risk of secondary infection should always be considered, so cleaning injuries from any invertebrate should be performed in a timely manner.
Envenomation
The venom of most scorpions is relatively harmless, and stings typically result in mild local inflammatory reactions. However, a small number of species are considered medically important due to the morbidity and mortality associated with their venom. Most of these species are in the family Buthidae, and include the genera Buthus, Androctonus, Leiurus, Centruroides, Tityus, Parabuthus, and Mesobuthus; species in other families include Hemiscorpion and Nebo.10,12 As a general rule of thumb, scorpions with relatively large pincers and small tails have less potent venom, while those with small pincers and large tails have more potent venom.12 Significant effects of more potent toxins involve the cardiovascular and respiratory systems.10
Centipedes are particularly aggressive creatures, and do not hesitate to bite. Envenomations are painful and often produce mild local inflammatory symptoms. Venom from large tropical species of the genus Scolopendra can cause more significant morbidity and sometime mortality.
Envenomations due to members of the insect order Hymenoptera (i.e., ants, bees, and wasps) are common and typically mild, unless anaphylaxis or other hypersensitivity occurs. Large numbers of envenomations can also potentially lead to serious medical conditions.
Noxious Chemicals
Many terrestrial invertebrates produce noxious chemicals compounds for defense. These can be secreted or forcefully ejected. Such substances are typically malodorous and highly concentrated.7 Identified chemicals in these compounds include acids, aldehydes, ketones, esters, hydrocarbons, lactones, phenols, benzoquinones, and terpenes.7 Myriapods species are particularly well known for producing dark brown substances from repugnatorial glands associated with each body segment that can cause a range of problems, from skin discoloration to blistering.3,15 Many insects are known to possess the ability to produce these noxious substances. The stick insect Anisomorpha buprestoides (also known as a "devil rider") is a common example in the Southeastern United States. Bombardier beetles (Brachionus spp.) are another well-known example.7
References
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18. Wirkner CS, Prendini L. Comparative morphology of the hemolymph vascular system in scorpions - a survey using corrosion casting, microCT, and 3D-reconstruction. J Morphol. 2007;268:401–413.
19. Zachariah TT, Mitchell MA. Invertebrate medicine and surgery. In: Tully TN, Mitchell MA, eds. Manual of Exotic Pet Practice. Saunders, St. Louis, Missouri. 2008:11–38.
20. Zachariah TT, Mitchell MA, Langan JN, Acierno MJ. Hemolymph osmolality of the Chilean rose spider (Grammostola rosea). Proc Rept Amphib Vet. 2010:117. (Abstr.)