The answer to your question is that an awful lot of terrestrial insects provide a fantastic source of nutrition for your fish. There are three things that you need to watch out for:
1) Is the insect producing something toxic - adult ants for example produce formic acid which has been said to kill certain fish, whereas ant eggs are a traditional fish food going back over 100 years.
2) Has the insect consumed something toxic - many caterpillars (aphids and other invertebrates) feed off plants that are toxic and will be gut loaded with these same toxins, and similar compounds that have formed though the process of digestion. Some insects will also have been exposed to insecticides and other anthropogenic or environmental contaminants, so you need to be happy about the source. Of course, if you think about tubifex collected from foul water then it is no surprise that this has been linked to fish disease due to the abundance and variety of toxins. However, most cabbages (legumes) and brambles are a safe bet. With all of these points you just need to think about "active" ingredients.
3) Can the fish digest it - Bloodworm can cause constipation and certain fish like Rams do not tolerate excessive feeding and will even die. Hard shells can get stuck in fish guts, so many beetles and isopods are only suitable for fish with the correct teeth, that are evolved to munch them down.
These three tests would apply to the following:
Grindal worms - an exceptional food because you can culture them and feed them high-quality multivitamin balanced cat food, and they pass these biological molecules onto fish.
White worms - ditto.
California black worms (cultured aquatic worms, but worth a mention here) - noted to revive angle fish from "fussy eating" habits. A great food when fish stop eating due to stress.
Earthworms - safe, tested, readily available. Suitable for human consumption.
Maggots - never tried this, but they are actually antimicrobial and therefore approved to treat open wounds and ear infections in humans. A very sanitary source would be required - i.e. controlled conditions because they stink when not bred properly.
Springtails - quite hard to master, but basically you blow or tap them over the water surface. Great for certain fish. No scent, eat odd scraps, and virtually effortless to culture.
Drosophila flies - a very interesting genus of flies many of which are flightless and easy to feed to fish. Very easy to culture, you can also tailor the food you feed to them in order to provide certain nutrients to your fish, like B vitamins.
Other flies - most are safe, but watch out for any that could provide a vector for other aquatic diseases, such as hoverflies, stoneflies, mosquitos etc. Think about the their full life cycle. Chopped lacewings are a good source of free food. Bluebottles feed off crap and will pass germs onto your fish - avoid coprophagous insects.
Spiders and similar soft bodied insects - Some can bite fish and most (including common garden spider) are so venomous that if they bit a human, that person would die. If a fish swallowed these venom glands, they could pierce fish guts. You therefore need to remove their head. I have been bitten by a cave spider and went hallucinogenic for 24 hours, I still have a scar, so be careful - cave spiders are territorial and aggressive. I saw my first mouse spider in the UK and it was the size of a tarantula - I'll never forget this. 14 or 15 UK species bite humans regularly. Septic shock is more dangerous than venom, so don't assume that you will just be okay. My scar took months to heal fully.
Isopods (including small woodlice) - check fish compatibility, there are also many aquatic varieties to choose. Very tasty if you are looking for something shrimpy to go in your pasta or sandwiches.
Aphids - excellent source of sugar for struggling lethargic fish; unparalleled in this respect.
Caterpillars - similar to aphids, but more prone to accumulate toxins depending upon plant. Some are also poisonous to deter predators, but some of the ones you get off prunus and legume plants I have found to be ideal. Brightly coloured ones were assumed to be more poisonous, historically.
Ant eggs - ideal and full of growth hormones. A delicacy throughout the world, best eaten raw, a good substitute for caviar.
Wasp grubs - ideal, a traditional fishing bait.
Earwigs, beetles, centipedes - they can bite fish. An earwig once attacked my leg when I was at home (I have an old and creepy house full of bugs and cave spiders). I got a nasty bite. Hard shells and only suitable for certain fish. Centipedes will sink and have venomous claws, also dangerous to handle.
Mealworms and small cricket cultures - I have no idea. You could to test them and please let us all know!
If you need help with any ID, then please just post on the forum. We have dedicated experts that can identify almost any bug, and give you advice.
It's good fun, and a lot safer to freeze your own foods because you know their source, and that they haven't been allowed to rot (I'm not a fan of stinking frozen commercial food because they poisoned my fish and a lot of cases you read about disease on here, seem to stem from these sources). We love - love - seeing pictures of what you have gone out to collect. It also minimises your carbon footprint, fuel costs, and is a healthy activity, great for children too.
Don't forget that you can collect high value pollen for shrimps from nettle flowers. Probably far superior to bee pollen in my opinion. You can also collect rowan berries, blackberries, elderberries, rose hips etc. - all are excellent for shrimp, fish, and snails, especially the latter. Some fish will eat berries whole. The maggots you get inside them are also ideal.
