Little-known sci-fi story read in 1976 (Proteus?): main character is caught by a giant orange spider thing which lays eggs in him and keeps him alive

All I can remember was the main character is caught by a giant orange spidery thing (not on Earth) which lays eggs in him and keeps him alive, he goes mad when the babies start eating him but survives.

Answer

It indeed seems to be Proteus (1979) by John Paton. Little is to be found on the Internet in English, but Google-translated from its Italian Wikipedia page:

Colonel Porter, officer of the Space Corps, has been abandoned for 5 years on the planet Conar, inhabited by the Drill population. When a war breaks out on the planet, the colonel is sacrificed by the Drill priests, who abandon him in a cavern infested by the Testarossa, arachnoid beings, who will use his belly to lay and incubate their eggs. Due to the injection of paralyzing substances, Colonel Porter finds himself being a passive spectator of the sexual rituals that the Testarossa perform on his body.

After a year spent in this terrifying condition, kept alive thanks to a nauseating substance that is directly injected into his throat by the Testarossa, he is rescued by some members of the Space Corps. His body is completely rotten and useless, but his brain, still fully functional, is removed and kept alive in a special structure. To avoid the damage caused by the absence of external stimuli, Colonel Porter's brain is constantly kept under the effect of sedatives, which will not allow him to conceive the condition in which he finds himself.

When it is discovered that the Drill possess the secrets of telophoresis, ie the ability of teleportation , there is a need to bring back to life Colonel Porter, the only man able to translate the writing Drill, now completely extinct due to a civil war . A team of doctors and scientists manages to connect the colonel's brain with an artificial body with human features, but with clearly enhanced physical abilities. After having touched the madness in the cave of the Testarossa, Colonel Porter refuses to return to Conar and flees from the hospital where his resurrection had taken place. Victim of a precarious psychic equilibrium, the fugitive will have no qualms about killing all those who hinder his freedom. General McShane, assisted by the team of doctors, will try by all means to capture Porter and bring him back to Conar, but this will prove to be a very difficult test, given the great resources of the new body of the colonel.

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Found with the Internet Speculative Fiction Database. The novel's page listed it being reprinted in Urania, an Italian SF magazine; I fed Google the Italian title and got the Wikipedia page, after reading the following on the ISFDb page for that Urania issue:

Proteus was translated by Beata Della Frattina as "La lunga morte del colonnello Porter".