Nature is not cruel, only pitilessly indifferent. This is one of the hardest lessons for humans to learn. We cannot admit that things might be neither good nor evil, neither cruel nor kind, but simply callous–indifferent to all suffering, lacking all purpose. – Richard Dawkins
Richard Dawkins made this in his book “The Greatest Show on earth” (I fantastic read BTW). I remember he wrote three examples in that book 1. Jealousy (Cuckoos pushing host bird’s eggs away from the nest ). 2. Slavery in Ant world 3. The cruel wasps. Here, more explanation on third one a piece by Stephen Jay Gould, pasting few paragraphs just for you.
The ichneumon, like most wasps, generally live freely as adults but pass their larva life as parasites feeding on the bodies of other animals, almost invariably members of their own phylum, the Arthropoda. The most common victims are caterpillars (butterfly and moth larvae), but some ichneumons prefer aphids and other attack spiders. Most host are parasitized as larvae, but some adults are attacked, and many tiny ichneumons inject their brood directly into the eggs of their host.
The free-flying females locate an appropriate host and then convert it into a food factory for their own young. Parasitologists speak of ectoparasitism when the uninvited guest lives on the surface of its host, and endoparasitism when the parasite dwells within. Among endoparasitic ichneumons, adult females pierce the host with their ovipositor and deposit eggs within. (The ovipositor, a thin tube extending backward from the wasp’s rear end, may be many times as long as the body itself.) Usually, the host is not otherwise inconvenienced for the moment, at least until the eggs hatch and the ichneumon larvae begin their grim work of interior excavation.
Among ectoparasites, however, many females lay their eggs directly upon the host’s body. Since an active host would easily dislodge the egg, the ichneumon mother often simultaneously injects a toxin that paralyzes the caterpillar or other victim. The paralyzes may be permanent, and the caterpillar lies, alive but immobile, with the agent of its future destruction secure on its belly. The egg hatches, the helpless caterpillar twitches, the wasp larvae pierces and begins its grisly feast.
Since a dead and decaying caterpillar will do the wasp larvae no good, it eats in a pattern that cannot help but recall, in our inappropriate anthropocentric interpretation, the ancient English penalty for treason — drawing and quartering, with its explicit object of extracting as much torment as possible by keeping the victim alive and sentient. As the king’s executioner drew out and burned his client’s entrails, so does the ichneumon larvae eat fat bodies and digestive organs first, keeping the caterpillar alive by preserving intact the essential heart and central nervous system. Finally, the larvae completes its work and kills its victim, leaving behind the caterpillar’s empty shell. Is it any wonder that ichneumons, not snakes or lions, stood as the paramount challenge to God’s benevolence during the heyday of natural theology?