Beetles are endopterygotes with complete metamorphosis.
Although beetle eggs are generally very small, their size, shape, color, and content vary extensively among species, as it is generally the case for most sexually reproducing species. A single female may lay from several dozen to several thousand eggs during its life time. Eggs are usually layed according to the substrat the larva will feed on upon hatching. Among others, they can be layed loose in the substrat (e.g. flour beetle), layed in clumps on leafs (e.g. Colorado potato beetle), or individually attached (e.g. mungbean beetle and other seed borer)or buried in the medium (e.g. carrot weevil).
The larva of a beetle is called a grub, and often represents the principal feeding stage of the life-cycle. Larvae tend to feed voraciously once they emerge from their eggs. Some feed externally on plants, such as those of certain leaf beetles and lady bird beetles, while others feed within their food sources. The larval period varies between species but can be as long as several years. All beetle larvae go through several instars, which are the developmental stages between each moult. In many species the larvae simply increase in size with each successive instar. In some cases, however, more dramatic changes occur. Among certain beetle families or genera, particularly those that exhibit parasitic lifestyles, the first instar (the planidium) is highly mobile in order to search out a host, while the following instars are more sedentary and remain on or within their host. This is known as hypermetamorphosis; examples include the blister beetles (family Meloidae) and some rove beetles, particularly those of the genus Aleochara.
As with all endopterygotes insects, beetle larvae pupate for a period of time, and from the pupa emerges a fully formed, sexually mature adult beetle, or imago. Adults have an extremely variable lifespan, from weeks to years, depending on the species.
Beetles may display extremely intricate behaviour when mating. Smell is thought to be important in the location of a mate.
Conflict can play a part in the mating rituals of species such as burying beetles (genus Nicrophorus) where conflicts between males and females rage until only one of each is left, thus ensuring reproduction by the strongest and fittest. Many beetles are territorial and will fiercely defend their small patch of territory from intruding males.
Pairing is generally short but in some cases will last for several hours. During pairing sperm cells are transferred to the female to fertilise the egg.
Parental care varies between species, ranging from the simple laying of eggs under a leaf to certain scarab beetles, which construct impressive underground structures complete with a supply of dung to house and feed their young.
There are other notable ways of caring for the eggs and young, such as those employed by leaf rollers, who bite sections of leaf causing it to curl inwards and then lay the eggs, thus protected, inside.
Beetles and their larvae have a variety of strategies to avoid being eaten, for example using camouflage to avoid being spotted by predators. These include the leaf beetles (family Chysomelidae) that have a green colouring very similar to their habitat on tree leaves. More complex camouflage also occurs, as with some weevils (family Curculionidae), where various coloured scales or hairs cause the beetle to resemble bird dung.
A number of longhorn beetles (family Cerambycidae) bear a striking resemblance to wasps. This defense, known as mimicry, can be found to a lesser extent in other beetle families, such as the scarab beetles.
Many species, including lady beetles and blister beetles, can secrete poisonous substances to make them unpalatable. These same species often exhibit aposematism, where bright or contrasting color patterns warn away potential predators.
Large ground beetles will tend to go on the attack, using their strong mandibles to forcibly persuade a predator to seek out easier prey.