Considering I have copies of the International Zoo Yearbook to hand, I figured I would get up relevant information here while I still have the books. In this case, I am uploading the species list of the then Insect (or more correctly Invertebrate) House from 1991. As well as the species list (with scientific names), I am including what the basic habitat type for each exhibit and the dimensions of the exhibit. The list is divided up by each of the themes in the old house; the species marked with a * are ones that have not bred in the collection.
Biomass
Italian honeybee Apis mellifera carnica - Observation hive with three frames (47×71×8cm)
Red wood ant Formica rufa - European pine forest with soil/peat (420×180×127cm)
Evolution
Upside-down jellyfish Cassiopeia andromeda - Marine (120×80×60cm/400 litres)
Land hermit crab* Coenobita clypeatus - Mangrove swamp (110×60×110cm)
African field cricket Gryllus bimaculatus - Top level of an African Plains split-level exhibit with shallow sand/soil (110×60×110)
Tanzanian red-legged millipede Epibolus pulchripes, Asian giant millipede Scaphistostrepus sp - Bottom level of the African Plains exhibit; a subterranean display with peat/leaf litter (110×20×20cm)
Feeding Strategies
Leaf-cutter ant Atta cephalotes - Stylized ant colony (585×76×50cm)
Giant blue centipede Rhysida afra - Individual glass tank with sand/moss (20×40×20cm)
African praying mantis Sphodromantis centralis - Individual glass tank with sand (20×40×20cm)
Assassin bug Platymerus biguttatus - Individual glass tank with sand (20×30×2cm)
Defence
Malaysian jungle nymph Heteropteryx dilatata, Macleay's spectre stick insect Extatosoma tiaratum - Tropical forest with peat soil (50×70×50cm)
Spiny stick insect Eurycantha calcarata - Forest floor with peat (20×40×30cm)
Sri Lankan bush cricket Vetralla quadrata - Tropical forest with peat (100×60×100cm)
Adaptability
Red-kneed bird-eating spider Brachypelma smithi - Scrubland with sand/soil (50×50×40cm)
Pink-toed bird-eating spider Avicularia avicularia - Bamboo forest with moss (50×70×60cm)
Brazilian bird-eating spider Lasiodora parahybana - Forest floor with moss/soil (50×40×50cm)
Sri Lankan tiger spider* Poecilotheria formosa - Human habitation with matting (50×40×50cm)
Fat-tailed scorpion* Androctonus crassicauda - Desert habitat with sand substrate (17×17×20cm)
Black widow spider Latrodectus mactans - Silhouette (17×17×8cm)
Madagascar hissing cockroach Gromphadorhina portentosa - Buttress root with soil/leaf litter(50×70×30cm)
Imperial scorpion Pandinus imperator - Jungle stream with moss (50×50×70cm)
African cave cricket Pholeogryllus geertsi - Cave with sand/soil (100×60×100cm)
African giant land snail Achatina/Archachatina sp - Tropical garden with sedge peat (110×60×100cm)
Silkmoths (various spp) - Moonlight/silhouette cylindrical net (80cm diameter, 90cm high)
Life Cycles
Damselflies (species unknown) - Jungle pond (320×236×100cm)
Desert beetle Tenebrionidae - Desert with sand (110×60×100cm)
Global Significance
Jewel wasp Ampulex compressa - Indian scrubland with sand + roosting material (110×60×100cm)
American cockroach Periplaneta americana - Household kitchen (110×60×70cm)
Migratory locust Locusta migratoria - Desert with sand substrate (110×70×100cm)
African dung beetle Scarabaeus semipunctatus - African scrub with sand/soil (100×75×100cm)
Silkworm Bombyx mori - Silk products/silk backdrop (50×50×40cm)
Conservation
Medicinal leech Hirudo medicinalis - Glass aquarium (120×50×60cm)
Olimpia's ground beetle Chrysocarabus olympiae - Alpine with sand/soil (30×40×40cm)
Red-kneed bird-eating spider Brachypelma smithi - Nursery/individual rearing tubes with vermiculite (10×7×10cm)
Postman butterfly Heliconius melpomene - Tropical forest walkthrough exhibit
British butterflies (various spp) - British garden in a seasonal outdoor exhibit
The off-show breeding areas also housed five species of Partula snail and wart-biter crickets Decticus verrucivorus together with further breeding space for Olimpia's ground beetles. Another article in the Yearbook from 1991 documents the arrival of a number of coconut crabs Birgus latro, of which the largest male was put on-show in the Insect House while the others remained off display.
Looking at the current 2018 stocklist, it appears about 20 invertebrate species from 1991 are no longer present in London Zoo.