「Insect unseen」refers to one that lives out of sight crawling under the skin or within the intestines, for a while at least, but its presence is surely felt, the BOTFLY, WARBLE FLY, GADFLY that infect livestock herds of meat asset hosts such as cattle:
「…the larvae of all species are internal parasites of mammals. Largely according to species, they also are known variously as warble flies, heel flies, and gadflies. The larvae of some species grow in the flesh of their hosts, while others grow within the hosts' alimentary tracts. The word 'bot' in this sense means a maggot. A warble is a skin lump or callus such as might be caused by an ill-fitting harness, or by the presence of a warble fly maggot under the skin. …Larvae from these eggs, stimulated by the warmth and proximity of a large mammal host, drop onto its skin and burrow underneath. …The equine botfly presents seasonal difficulties to equestrian caretakers, as it lays eggs on the insides of horses' front legs, on the cannon bone and knees, and sometimes on the throat or nose, depending on the species. …When a horse rubs its nose on its legs, the eggs are transferred to the mouth, and from there to the intestines, where the larvae grow and attach themselves to the stomach lining or the small intestine. …The human botfly occasionally uses humans to host its larvae. The larva, because of its spines, can pose an extremely painful subepidermal condition. …In cold climates supporting reindeer or caribou-reliant populations, large quantities of Hypoderma tarandi (warble fly) maggots are available to human populations during the butchery of animals. Some people still relish these maggots as seasonal luxuries containing high levels of proteins, fats and some minerals. Copious art dating back to the Pleistocene in Europe confirms their consumption in premodern times as well.」
[ https://en.wikipedia.org/wiki/Botfly 2013-09-14 ]
The reference to a「horse fly」relates to a fly that bothers horses, and not to the specific animal called a horsefly in English…horseflies are neither dead nor unseen, nor do they or their maggots prey on dead meat:「Female horse and deer flies feed on the blood of large mammals, including livestock and humans. Male horse and deer flies feed only on nectar.」
[ https://www.uky.edu/Ag/CritterFiles/casefile/insects/flies/tabanid/tabanid.htm 2018-04-26 ]

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: horsefly; gadfly
Japanese EDRDG: gadfly, horsefly
Unihan extended: horsefly, gadfly