I've started putting together an article on feeder insect nutrition but it's just getting started. Here's what I've got so far in draft and lots of linked sources. Keep in mind that I have not proof read this yet so there could be mistakes, typos, etc.
Calcium, Availability, and Gutloading
Insects are invertebrates and most species do not contain very much calcium compared to vertebrates which have large amounts of mineralized bones (Finke & Oonicx, 2014). However, some species such as isopods and black soldier fly larvae do have mineralized exoskeletions (Finke & Oonicx, 2014). I have noted that some of the salamanders I fed black soldier fly larva to would sometimes pass the exoskeleton largely undigested. Finke and Oonicx (2014) note that mountain chicken frogs digested only 44% of black soldier fly larvae fed to them live, but that number went up to 77% when the larvae were mashed. The calcium levels of many feeder insects can be raised through gutloading. Research has shown that crickets fed calcium enriched diet contain significantly more calcium than crickets fed the same diet with less calcium (Finke, Dunham, & Kwabi, 2005). Diets with 65g Ca/kg and higher of calcium were found to produce insects with significantly higher calcium content. Finke also found that most commercially available gutloads that he tested did not actually contain the amount of calcim listed on the label. Particle size may also play a role as a finely ground diet with 65 Ca/kg versus a thicker particle size diet with 103g Ca/kg did not result in the proportionate increases in calcium content that one might expect. The higher 103g Ca/kg diet produced small cricket nymphs with only 8% higher calcium amounts and adult crickets with 30% higher calcium amounts (Finke et al., 2005). Based on the mixed results of actual contents of commercial gutloads and the impressive results from gutloading with a calcium enriched feed, I would recommend reptile keepers boost their cricket gutload to at least 65g Ca/kg 48 hours prior to being used as feeders. Such high calcium diets may affect the normal lifecycle of insects, thus the recommendation to feed them the special diet in a separate, small contain 48 hours prior to feeding them to reptiles. Costs to reptile keepers will be minimal as calcium is inexpensive.
Caroteniods, Vitamin A, and Insects
Caroteniods are found at higher levels in wild caught insects than captive reared (Finke & Oonicx, 2014). It is unclear which insectovores can convert these caroteniods into Vitamin A (Finke & Oonicx, 2014). In many insects these caroteniods are stored in the eyes of the insects and larval forms tend to have less eye material than adult forms. However, increasing caroteniod content in fruit fly medium did result in adult flies with increased caroteniod content (Finke & Oonicx, 2014). Dugas, Yeager, and Richards-Zawacki (2013) added 1.4g spirulina, .7g powdered marine algae, and .7g red phaffia yeast as a carotenoid source to 16.7g dehydrated potatoes, 3.7g Brewers yeast, 4.9g Confectioner's sugar, 2.7g Methlparaben, .3g bakers yeast, and 80g tap water. Pairs of dart frogs fed on the carotenoid supplemented diet produced fewer clutches but more tadpoles and metamorphs than pairs kept on an unsupplemented diet (Dugas, Yeager, and Richards-Zawacki, 2013).
Other Vitamins and support for a varied diet
Finke & Oonicx note that Vitamin E levels appear to be low in captive reared insects and the diet may be a factor as different studies have shown different amounts even for the same species (2014). However, butterworm larvae, silkworm larvae, fruit flies, and house flies all tend to contain higher levels of vitamin E (Finke & Oonicx, 2014). With Vitamin B1, both wild and commercially reared insects varied greatly in thiamine content. House crickets, butterworms, red runner roaches, and superworms had low levels of thiamine, while mealworm larvae (but not adults), waxworms, silkworms, adult house flies, and black soldier fly larvae contained higher levels.
Chitin
Look on any lizard or amphibian forum and you'll soon see discussions about chitin content in various insects. Unfortunately, much of the information on the subject found on the Internet appears to be incomplete. Finke (2007) found the chitin content actually appeared to be higher as measured in mg/kg in crickets than in giant mealworm larvae or waxworms. Furthermore, Finke (2007) found that the hardness of insects exoskeletons appeared to be related to factors other than chitin content, such as a higher concentration of ammino acids. In my own personal experience, I have found that if a particular insect exoskeleton can truly not be digested, you will see that in the feces. It seems that the digestability of insect exoskeletons is more complicated than whether or not they contain a certain amount of chitin.
Dugas, M. B., Yeager, J., & Richards-Zawacki, C. L. (2013). Carotenoid Supplementation Enhances Reproductive Success in Captive Strawberry Poison Frogs (Oophaga Pumilio). Zoo Biology, 3, 655–658.
Finke, M. (2007) Estimate of Chitin in Raw Whole Insects. Zoo Biology, 26, 105–115.
Finke, M. D., Dunham, S.U., & Kwabi, C.A. (2005). Evaluation of Four Dry Commercial Gut Loading Products for Improving the Calcium Content of Crickets, Acheta domesticus. Journal of Herpetological Medicine and Surgery, 15(1),7-12.
https://doi.org/10.5818/1529-9651.15.1.7
Finke, M., & Oonicx, D. (2014). Insects as Food for Insectivores. In J.A. Morales-Ramos (Ed.), Mass Production of Beneficial Organisms (pp.586-616). Cambridge, MA: Academic Press.
, vitamin A, etc
