Beetle larvae live in almost all biological niches imaginable. Some live as predators in leaf-litter, under tree bark, and in fresh water, and even on flowers.
Predators, of course, move well and have well developed sensory organs to find their prey.
Leaf beetle larvae: cryptic (left), aposematic (middle), actively disgusting holding excrement package (right)
The ones that are chewing away openly on the outside are usually protected by cryptic, mimetic or aposematic colors, shapes, and or behaviors. Some even build their own protective cases.
Beetle larvae may be storage pests and raid organic material horded by squirrels, pack-rats, and humans. This includes starchy products as well as fur coats and insect collections. But as far as I know, no beetle species, larvae or adults, directly parasitises any living warm-blooded animal.
Endocrinological control of larval development:
To grow, beetle larvae have to molt out of their rigid chitinous skin to a 'larger size'. This happens several times until finally the larvae changes into the stiff, immobile pupa from which eventually the adult beetle emerges.
Delicate shifts in the balance of three hormones determine whether a molt leads from one larval stage to just another, larger one, or to pupation and thus metamorphosis.
In short, Prothoracicotropic Hormone (PTTH) is released from the brain and activates the prothoracic glands to release Ecdysone. Two endocrine glands (corpora allata) in the head of the larva are producing Juvenile Hormone (JH). An increase in Ecdysone while the JH levels in the heamolymph of the larva are high will cause molting into another, larger larva. An increase in Ecdysone when the levels of JH are low will cause the larva to molt and undergo pupation and metamorphosis instead. PTTH release and corpora allata activty (JH production) are controlled by the central nervous system that is able to integrate endogen and exogen inputs (body size, age, food availability, day length etc.)
Pupa of the Palo Verde Root Borer (Derobrachus hovorei)
But back to our 'Young Beetle':
On a Mount Lemmon trip of the Sabino Canyon Naturalists led by Ned Harris, we actually found what could really be called a 'young beetle'. It was Mid-August an it had been raining. Even the mountain climate was humid and warm enough to be called muggy. The area along the Oracle Ridge trail was heavily burnt several years ago. The dead trees are still upright and soot and carbonized parts have been eroded away. These are ideal conditions for many different fungi and the mushroom connoisseur among beetles, Gibbifer californicus, the Pleasing Fungus Beetle.
First we found some adults and last instar larvae, and then a whole gallery of hanging pupae. About 60 of them were lined up under the protective overhang of a leaning tree. Some where already empty.
Perched on top of pupae and empty cases was a ghostly white beetle with his under-wings partly unfolded.
This very freshly eclosed Fungus beetle showed nothing yet of the characteristic blue coloration or any of the black accents. This was a teneral specimen, a true young beetle. His final colors became apparent only hours later when his new exoskeleton had completely dried.
The mature color of Gibbifer californicus ranges from gray to deep blue (left) and purple. However, the beautiful color does not survive the death of the beetle. Gibbifer californicus specimen in collections are of a sickly yellow hue, only the black extremities remain dark.
Incidentally, the illustration in the Peterson Field guide (right) depicts a beetle with those non-colors to represent the species. That image was one of the triggers that started my collection of life-images of Arizona beetle species.