Life Cycle and Reproduction

Firebrats, which on average have a two to three year lifespan, become sexually mature adults in anywhere from 3 to 24 months from hatching.  The immature, or nymph, stage of Thermobia domestica is very similar to the adult; there is no metamorphosis between the two stages.  From hatching as a small egg, usually no larger than 1/25 inch, they grow through a nymph stage right into adulthood where they may reach 1/2 inch in length.  Though they molt during these stages, there is no true metamorphosis which takes place, and it takes several molts before the adult scales develop. (Davies 1988)

Molting is a bit of a curiosity in the firebrat.  Most insects not only metamorphose, they also cease molting once they reach adulthood or become sexually mature. (Hickman et al. 2009)  This is not the case with Thermobia domestica who tend to molt through their entire lifespan.  Molting occurs in these insects through adulthood, and though series of cellular changes occur there is no metamorphosis or cessation of molting to cue the adult stage. (Lanham 1964) 

Reproduction coincides with molting in a number of ways.  Mating is restricted to the molting cycle, and males can only proposition females whose molting cycles are within three (in rare observed cases maybe four) days of the male's cycle. (Nijout 1994)  It has been observed that firebrats mate with each and every instar (period between molts) and females lay eggs after every molt following.  Females, who double in size after the onset of reproduction, will lay eggs in batches of as many as 50 or 60, and will produce up to 100 or more batches in a lifetime. (Lanham 1964)  Being the flat and small organisms they are, firebrats will lay eggs in tiny cracks or crevices where they are secluded from disturbance.  The eggs take less than two weeks to hatch, and aside from size and scales, nymphs (or juveniles) look like the adult. (Davies 1988)

One of the most interesting characteristics of Thermobia domestica is their mating dance.  Once it has been determined a female is in the appropriate stage of an instar, a male will approach her head-on repeatedly while shaking his head from side to side. (Lanham 1964)  He will sometimes come close enough to touch her head, but not necessarily with every pass.  The male will then whirl in a circular motion, up-twisting his abdomen and turning his head.  The male will next walk past the female, intentionally brushing her legs with his own, to which the female responds by moving forward a small distance and turning her body around.  From this point the dance repeats until the male is prepared to lay a spermetaphore (sperm sac), which in one recorded case took half an hour!  When ready, the male lays one singular spermetaphore on the ground just in front of the female.  He speeds past her then, again intentionally brushing her legs and causing her to move forward.  Only in this last dance, the female moves directly over the sperm, and it is seized by her genitalia.  This quirky ritual has been likened to the mating dance of spiders, and one author compares the entire reproductive process to that of artificial insemination in humans. (Ordish 1960)