Arachnophobia

Entomologist James Atherton (Julian Sands) searches the Amazon rainforest with the hope of discovering new species of insects and arachnids. Descending into an enormous sinkhole, his team sets up collectors on the ground below a large tree, then blow smoke up into the canopy of the tree. A large number of bugs fall into the collector, including a very aggressive new species of spider. The spider is captured and chloroformed for research; and is later revealed to be lacking sex organs, thus making it a drone, or soldier, highly unusual in an arachnid species. A nature photographer, Jerry Manley (Mark L. Taylor), who has been suffering a fever while traveling with the team, unknowingly has a fertile (non-drone) male spider of the same species jump into his backpack, that night sneaking into his sleeping bag and biting him. Manley has a massive seizure from the venom and dies. The remaining scientists takes his body back to the United States, blaming Manley's death on the preexisting fever. The spider follows the crew, jumps into the stretcher carrying Manley's body, and is sealed into a box with the corpse.
Manley's body arrives at the funeral home in his small town of Canaima, California, and mortician Irv Kendall (Roy Brocksmith), stunned at the body's greenish color (from the spider's digestive fluids) does not notice the spider inside the coffin when he opens it. The spider sneaks outside and is picked up by a raven, but before the raven gets back to its nest, the raven is bitten by the spider and it falls dead to the ground in front of the barn of the Jennings family. Ross Jennings (Jeff Daniels) is a family physician who had moved to the town from San Francisco and who faces a lack of patients due to elderly rival Sam Metcalf (Henry Jones), who was supposed to retire and shift his patients to Ross, but decided to maintain his practice.
The Amazonian spider mates with a female domestic house spider and makes a nest in Jennings' barn, producing hundreds of infertile drone offspring, all of which have their father's lethal bite. Ross, along with his son Tommy, has arachnophobia, making them targets of ridicule to wife Molly (Harley Jane Kozak) and daughter Shelly (Marlene Katz).
Ross' first patient, Margaret Hollins (Mary Carver), dies after being bitten. The town's residents believe that she died from a heart attack, despite Ross' suspicions that something else was at work since her body appeared to have had a seizure instead. After a spider also kills a football player, a death that is attributed to a football injury, Ross is known to the town as "Dr. Death", having all his patients dying after having seen him. When Doc Metcalf is bitten and dies, Ross suspects that deadly arachnids could be infesting the town.
Ross and county coroner Milton Briggs (James Handy) perform an autopsy on the victims and confirm Ross' suspicion that the deaths were caused by spider bites. Atherton arrives in town with his assistant Chris Collins (Brian McNamara), joining Briggs, Sheriff Lloyd Parsons (Stuart Pankin) and exterminator Delbert McClintock (John Goodman) in the spider investigation. They discover that the killer spiders have a short life expectancy due to being a mixed breed. Atherton tells them that the spiders are soldiers, sent out to eliminate potential threats for the general male spider. He also learns that the general spider produced a queen, which it likely mated with to produce a second nest someplace, guarded by the queen, which could produce fertile offspring. Atherton elaborates that, unlike the sinkhole of the spider's original habitat, which limited its territory, the transplanted spiders can progressively expand their territory, possibly culminating in their worldwide dispersal. The group sets out to destroy both nests and kill the queen and general, but Atherton plans to capture the general alive for his agenda after discovering where the first nest is located. Atherton plucks at the web to lure the general out, but gets caught by surprise as the general leaps from his hiding place and fatally bites Atherton.
Ross, Chris, and Delbert eventually discover that the nest is in Ross's barn, and Ross sends Delbert to destroy it, where the exterminator finds Atherton dead before destroying the nest. Chris gets the Jennings family out of their infested house, but Ross falls through the floor into his wine cellar, which turns out to be the spiders' second nest, containing fertile offspring. After electrocuting the queen, Ross battles the general while also attempting to burn the second egg sac. He becomes trapped underneath fallen debris, with the general about to deliver the killing strike when Ross flings the general into the fire.
As the egg sac hatches, the general jumps out of the fire. Ross shoots him with a nail gun and the projectile sends the burning spider into the egg sac, destroying the nest with fire and ending the plague.
Having enough of the countryside along with the near-death experience, the Jennings family immediately moves back to San Francisco, appreciating city life once more, despite minor earthquakes disrupting them.

Question: What was in the coffin with Manley's body?
Answer: A spider.