Leafminers (Liriomyza spp.) are dipterous insects, just like the common housefly. In Europe three species of Liriomyza occur: the tomato leafminer (Lyriomyza bryoniae), the serpentine leafminer (L. trifolii) and the pea leafminer (L. huidobrensis). In these three species, adult females measure 2-3 mm. They are black with yellow, having a conspicuous yellow spot on their back. Only specialists are able to distinguish the three leafminer speces. Males are a little bit smaller (1.5 mm).

With her barbed ovipositor, a female leafminer pierces holes in the upper surface of the leaf to extract plant sap (feeding spots). Males do not have an ovipositor, so they make use of the feeding spots made by females for their food. In such e puncture a female can also deposit an egg. Feeding spots are round and egg spots are oval.

The egg hatches into a tiny fly larva (maggot), that immediately starts eating its way through the leaf. There are three larval stages. In the first stage, the larva is transparent, but later, depending on the species, it turns dirty-white to yellow-ochreous (L. trifolii). Just before pupation, the larva cuts a sickle shaped hole in the leaf cuticle and wiggles its way out. It usually lets itself fall off the leaf to pupate in the soil or between the folds of the plastic in case of substrate culture.

Sometimes, however, the pupae stay hanging on the leaf. Depending on the species, the pupa is yellow to (reddish) brown. The development time from egg to adult depends largely on temperature. For the serpentine leafminer, it takes 12-14 days at 30°C (86°F) and 54-61 days at 15°C (59°F). The first generations at the beginning of the cropping season often come in waves.

An adult female lives for 1-2 weeks. The number of eggs she deposits depends on the species, the host plant and on temperature. It can vary from several tens to some hundreds.