The prothoracic glands are either of a pair of

endocrine gland The endocrine system is a network of glands and organs located throughout the body. Along with the nervous system, it makes the neuroendocrine system, which controls and regulates many of the body's functions. Endocrine glands are ductless gland ...

s located in the

prothorax The prothorax is the foremost of the three segments in the thorax of an insect, and bears the first pair of legs. Its principal sclerites (exoskeletal plates) are the pronotum (dorsal), the prosternum ( ventral), and the propleuron (lateral) on ...

of certain insects that regulate

molting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at ...

. They have an ectodermal origin and secrete

ecdysteroid Ecdysteroids are arthropod steroid hormones that are mainly responsible for molting (ecdysis), development and, to a lesser extent, reproduction; examples of ecdysteroids include ecdysone, 20-hydroxyecdysone (ecdysterone), turkesterone and 2- ...

s, such as

ecdysone Ecdysone is a prohormone of the major insect molting hormone 20-hydroxyecdysone, secreted from the prothoracic glands. It is of steroidal structure. Insect molting hormones (ecdysone and its homologues) are generally called ecdysteroids. Ecdy ...

and 20-hydroxyecdysone. They usually disappear in adults.Klowden, M. Physiological Systems in Insects Endocrine Systems.
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References

Insect anatomy Arthropod glands {{Insect-anatomy-stub