INVERTEBRATES are those animal species which do not belong to the Sub-Phylum Chordata, creatures with backbones. Put crudely, invertebrates lack a dorsal spinal chord with the nerves running through it and the bony or cartaliginous skeleton possessed by fish, amphibians, reptiles, birds and mammals. Despite this apparent disadvantage, a staggering 95% of living creatures on the earth are invertebrates, from single-celled creatures such as amoebas (admittedly a rather borderline case for the animal kingdom) through sponges, anemones, jellyfish, various parasitic worms, annelid worms, arrow worms and other little-seen creatures up to the insects, the arachnids, the crustaceans and the molluscs, two of whose representatives, the octopus and the squid, are actually fairly intelligent and have developed certain features (such as the form of the eye) normally found only among vertebrates.
Apart from the sheer number of invertebrate species, their diversity is enormous. Molluscs, for example, form a zoological phylum of their own and range from the sedentary brachiopods and mussels up to the giant squid. They number about 300,000 different species but this number pales into insignificance besides the insects, of whom at least 800,000 species are recognised, a total that in truth is probably at least one million. Most if not all of these families are also very old if not ancient, some with roots going back to the Cambrian era of about 500 million years ago. The horseshoe crab Xanthippus is a living relative of the once-abundant trilobite and is at least 300 million years old. Millipedes and arachnids, in the form of spiders and scorpions, conquered the land some millions of years before the first lungfish appeared. Insects are actually more recent to the history of life on earth but are now so hugely significant that much of life depends directly or indirectly on their activity, both as items in the food chain and as pollinators of plants.
The term 'spineless' has become one of derision in human society when applied to a human being, but in fact most of the invertebrates have developed their own form of body structure which grants them stability and structure. Worms tend to have a protective cuticle around them, while arachnids, crustaceans and insects rely on an exoskeleton, in essence having a hard shell or outer layer which is periodically shed as they grow. Marine invertebrates tend to rely less upon such structures, the water supporting them, but even here lobsters and crabs have exoskeletons while cephalopods usually have a cartaliginous pen, a sheath-like structure inside their body, that gives them internal support.
David Attenborough chose an apt title in Alien Empire, for the way of thinking of virtually all invertebrates is truly alien to the vertebrate mind. This is not so much a condemnation as a simply fact. Vertebrate brains are a large complex of cells formed at the frontal end of the spinal column, differing in their makeup from class to class but enormous in contrast with invertebrates, some of whom have only a sort of 'ring main' of nerves at the front of their bodies. Even arachnids and insects are fairly undeveloped in this area. Yet social insects also display a sort of 'hive consciousness' that as yet we do not fully understand, including organisation of their colonies into a social hierarchy and the construction of large and often intricate structures such as anthills and termite mounds. Furthermore octopus have shown the ability to use deductive processes, eg uncorking a bottle to get at the shrimp inside. While the concept of personality in, say, termites may be perhaps limited, a number of writers have commented on the apparently different personalities of individual land hermit crabs, for example.
Although there are over a million invertebrate species on earth, a relative handful are kept by enthusiasts. Heading the list would have to be tarantulas and non-dangerous scorpions, followed by stick insects, millipedes and centipedes, hissing cockroaches, land hermit crabs and giant African land snails. Reptile and amphibian keepers also maintain colonies of such creatures as grey or brown crickets, occasionally breeding them, for a food supply for their vertebrate pets. It is notable that herpetologists (those who keep reptiles and amphibians) tend also to be interested in invertebrate creatures, an interest not often displayed by people that keep the more usual mammalian pets such as cats and dogs. I hesitate to use the expression "invertebrates as pets", because most invertebrates cannot be treated in the same way as cats, dogs, rodents or reptiles. Although some will crawl up your arm, most have little wish to be handled, far less so than reptiles or amphibians, let alone a cat, dog or rabbit. Handling some can actually create problems, for example a tarantula flicking hairs into your eyes or a scorpion stinging you, albeit mildly. In nearly all cases invertebrate captives are display creatures, to be admired for their colours, different body form and "alien" factor, rather than kept as companion animals.
If you choose to keep invertebrates, they do have some advantages over other animals provided you are aware that they are not companionable. Apart from the very few colossi like the giant squid, most are small enough to be kept in a small tank. Arachnids such as spiders and scorpions are notoriously inactive for most of their lives and do not need a lot of space to exercise. Being cold-blooded, most invertebrates also have fairly modest food requirements: stick insects just require a piece of fresh bramble, fo example. Some will breed readily, in some cases almost too readily! Most do not vocalise or make any noise, and few have the requirements for UV light demanded by lizards and tortoises.
Apart from their lack of interest in their keepers, there are other factors that you should be aware of when considering invertebrates. The most obvious one is don't buy anything that is dangerous. It is a sad fact that certain people are turned on by the idea of keeping something highly venomous or otherwise potentially dangerous in their homes. In some ways an escaped venomous scorpion or spider is more dangerous than an escaped python or cobra, since an escaped snake can more easily be traced (and that's hard enough). A spider or scorpion can fit into small spaces and move about silently. If it is fertile then it can lay a good many eggs, which should they hatch could present you with a vast problem. There is no real need to keep dangerous species, especially if you are a beginner: the most attractive scorpions, for example, are the least dangerous, whereas the truly deadly ones are rather nondescript small straw-coloured creatures.
The other potential problem for a would-be keeper is getting the right information on a species. Fortunately there are now a good number of books available at pet shops or over the Internet, particularly with regard to the more common invertebrate pets I listed above and especially on tarantulas and scorpions. You should ideally read one of these books (borrow one from your local library if you're short of cash) before trying to acquire an invertebrate. That way you will know how to keep it, what it eats and whether you actually can keep it, practically speaking.
I don't keep any invertebrates as pets myself, but I have a number of friends who do and I have become aware of them as interesting creatures in their own right. Therefore any information offered here will be a collation of data, facts and anecdotes from other sources rather than personal experience. I am also pleased to be able to offer an index of periodical and magazine articles on various invertebrates that have appeared over the last few years.
Index of Magazine Articles on Invertebrates - an index of articles on various invertebrate species taken from herpetological magazines over the past few years. Please note that this index will be added to fairly constantly and many articles are still to be indexed, so if you don't see what you want, come back again over the next few weeks!