The animals I keep predominantly eat various invertebrates. Some are omnivorous and a very few are exclusively vegetarian. There are many sites which offer excellent dietary advice for the vegetarian species and in this respect I follow the experts in these species. As an example I would recommend Terry Thatcher's excellent Uromastyx care sheet which can be found here. I also keep a small number of snakes which are fed on (dead) mice.
Since my "specialist subject" amphibians and reptiles are invertebrate eaters I will concentrate here almost exclusively on how and what I feed them with.
One of the most common problems with captive lizards in particular is obesity. I try to ensure that all of my animals are at least a little bit hungry apart from when they have just eaten or if they are growing young. What this means in terms of frequency is, of course, something that varies according to season, life-stage and species. Having said this, in most species feeding behaviour in the wild can best be described as little but often and it is best to emulate this so far as possible. Accordingly, whenever I can I give the animals in indoor vivaria small amounts of food on a daily basis. I try to ensure that it is not so much that they would not be prepared to stuff themselves with more! The obvious exceptions are hatchlings/newly metamorphosed animals. Since they convert almost all of their food into growth these are fed enough to satisfy them each day.
Animals kept in outdoor vivaria whether open or covered (greenhouses) are a different matter as they have access to a certain amount of naturally occurring food and their eating behaviour is far more governed by natural weather conditions. In practice, I find that feeding these once or twice a week is all that is necessary.
Occasionally circumstances such as being away mean that these feeding regimes cannot be adhered to. In practice this is normally no more than three to four days. This is of little consequence to the outdoor animals and with those indoors I simply ensure that they are given maximum amounts of food before and after the absence. When, however, I have young being reared, I ensure that someone does feed them every day.
Generally speaking water is far more critical and it is important to ensure that adequate water for the species is always available. For many that simply means ensuring that a clean bowl of water is available. Others, however, may need daily spraying.
So far as snakes are concerned feeding frequency is somewhat more fluid. They tend, and in fact are designed, to have substantial meals at (compared to lizards) much wider intervals. The frequency can also be affected by the size of the food items. In my case, feeding is at approximately 1 to 2 week intervals, which appears to work well judging by the condition of the snakes.
The vast majority of the invertebrate eaters will only eat live and moving food. Fortunately, suitable food insects and other invertebrates are readily available and many one can breed oneself. This cannot be an exhaustive list but the following does cover some of the available options.
Waxworms (Galleria mellonella). These are the larvae of a form of moth. They are the scourge of beekeepers as they can infest hives, the larvae eating the honey. Their life cycle results in small sizes suitable for hatchlings to large - about 12mm long - and all stages between. They are very rich in sugars and fats and most insectivores love them BUT they should be used very sparingly. They can particularly benefit females before and after egg-laying and will often restore an interest in food in an animal that is not eating well. They should be dusted with a suitable multi-vitamin.
Mealworms (Tenebrio mollitar). These are a beetle larvae and a long standing terrarium standby. They can be purchased in two sizes "mini" suitable for most hatchlings and "regular". They are reputed to cause diarrhoea and to be largely indigestible due to their chitin. In practice, however, there are no problems IF they are gut loaded by feeding with green stuff and (especially) carrots and dusted with a suitable calcium rich multivitamin. They are a very good food source, rich in protein and thus particularly suitable for encouraging growth rather than boosting energy.
Buffalo Worms (Alphitobius spp( laevigatus/diaparinus?)) These are a small beetle larvae ideal for hatchlings They are becoming difficult to obtain and are also difficult to maintain for long periods (but only needed for first two weeks of most species' lives). They are very slightly hairy allowing good adhesion of vitamin powders and will eat anything - thus ideal for gut-loading. Very good food source, high in protein but reasonably well balanced if gut loaded and dusted with a suitable multi-vitamin powder.
“Super-giant” mealworms (Zophobas morio). Another, but much larger beetle larva. Medium to large lizards will eat these. Nutritionally they are very similar to mealworms and require gut loading and dusting similarly. Due to large size the digestible (flesh) to indigestible (chitin) ratio greatly improved. (Body Mass v. Surface Area)
Crickets. These are another long standing terrarium food. Hatchling (often sold as "Micro" or "Pinhead") crickets are suitable for the smallest of hatchlings or newly metamorphosed animals, while the adults of the largest species will provide a tasty snack for even the larger Agamids and Iguanids. Three varieties are available:-
House Cricket (Acheta domestica)
Banded Cricket (Gryllodes sigillatus)
Black (Field) Cricket (Gryllus bimaculatus)
They are rather fatty, must be gut loaded with green stuff, carrots etc and dusted with a suitable multivitamin.
Locusts (usually Schistocera gregaria). These are readily available but tend to be expensive. Nevertheless they are an excellent food source and should be used even if only as an occasional treat. The smallest sizes will suit many small animals including larger hatchlings while fully grown ones are suitable for the largest of insectivorous lizards. Gut loading should occur with proper locust maintenance (i.e. feeding with green stuffs) but still need dusting with multivitamin. They have an excellent balance of various dietary requirements
Phoenix Worms(Hermetia illucens). These are the larvae of the Black Soldier Fly. They have only recently become available in the UK. They have, purportedly, the best calcium/phosphorus ratio of any commercial feed insect. The range of sizes from a mere 1.5mm long to around 18-20mm makes them suitable for most animals. In addition, most animals seem very attracted by "maggot-like" prey - and these certainly are. They are easy to maintain at room temperature and actually deter houseflies! Should they pupate and adult flies eventually emerge these are large clumsy fliers and eminently suitable for Tree Frogs and the like.
Sweepings and other naturally occurring live foods. Hedgerows, fields and gardens abound with invertebrates. Particularly favoured examples are Spiders, Grasshoppers and Shield Bugs, but most small insects captured in a sweep net will be taken. In addition, most amphibians and reptiles appear to love earthworms of any species. Size appears to be almost irrelevant as they are easily compressed and will often be torn apart by two or more lizards. Worms are also exceptionally good in terms of gut content and calcium/phosphorous ratio. Lobworms (the typical large garden worm in the UK) can be cut into three for smaller species and can be obtained from specialist fishing bait companies at a very reasonable price. Slugs and snails are, of course, very common. Many amphibians will eat slugs as will many lizards and the larger species of lizard and terrapins will eat both with immense pleasure. These naturally occurring food sources are excellent in that they have a varied nutritional values and even dusting is probably not necessary. BUT, great care must be taken to ensure that they have not been exposed to pesticides, slug pellets and the like. If you are in any doubt DO NOT use them.
Rodents (http://frozenreptile.co.uk/). Frozen mice and, less commonly, frozen rats, can be bought from some petshops although far better value for money and greater range can be found online. Rodents are the only suitable readily available food for the vast majority of snake species in captivity. All of these are available in a variety of life stages suitable for the smallest snake or carnivorous lizard right up to all but the largest constrictors. Most smaller species of lizards do not eat rodents of any size, but the larger Lacertids such as Eyed Lizard adults and certainly monitor and tegu species will. Note should be taken of the animals natural food range. Thus, for example, an adult Timon will eat occasional small rodents or hatchling birds while animals such as monitors and tegus are true carnivores and such food items will need to form a much large proportion of their diet. Obviously these foods should be throughly defrosted before offering them to the animal. It should be noted that it is illegal in the UK to feed live rodents to snakes or lizards. We recommend "Frozen Reptile" at http://frozenreptile.co.uk/ as a reliable source of good value, ethically produced and good quality frozen foods.
Other foods. Many predominantly insectivorous species will also eat a small amount of green stuff. This is notable among the Agamas by most of which a little Curly Kale in particular is much appreciated. Generally speaking Lacertids are less likely to eat plant matter but some will take the occasionally small soft fruit such as Blackberries. The larger species such as Eyed Lizards seem to love Dandelion flowers. Amongst Lacertids the most exceptional behaviour is seen in certain Island species such as Lilford's Wall lizard and the various Galloti species from the Canary Islands. These all seem to be truly omnivorous, their absolute favourite being banana - but beware, these have an extremely poor Calcium/Phosphorous ratio. These lizards are seldom available due to their status in the wild.