Hisser Lab
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A butterfly changes completely on its way to adulthood. A hisser does not. This is the research on the two great patterns of insect growth, and why the contrast teaches so well.
At a glance
A hisser grows by incomplete metamorphosis. It passes through three stages, egg, nymph, and adult, molting about six times along the way, and a young nymph already looks like a small wingless version of the adult. There is no caterpillar and no pupa or chrysalis, which is the big difference from a butterfly. This pattern is normal for cockroaches and for many other insects.
- Growth pattern
- Incomplete metamorphosis (hemimetabolous)
- Stages
- Egg, nymph, adult
- Nymphal molts
- Six (six nymphal instars)
- Egg incubation
- About 60 days inside the mother
- Time to adult
- About five months
What are the stages of a hisser's life?
The Madagascar hissing cockroach (Gromphadorhina portentosa) has three life stages: egg, nymph, and adult.[1][2] The female keeps her eggs inside her body in an egg case (an ootheca) for about 60 days, then the young hatch and emerge as live nymphs.[2][1] A newly hatched nymph already looks like a small, wingless version of the adult, not like a grub or a caterpillar.[1] The nymph then grows through six nymphal instars, shedding its skin six times, with the last molt turning it into a mature adult.[1][3] The whole trip from hatching to a breeding adult takes about five months.[1][2] Adult hissers stay wingless, so the final molt does not add wings; it just produces a larger, sexually mature roach.[1]
How is this different from a butterfly?
A butterfly goes through complete metamorphosis, with four stages: egg, larva (caterpillar), pupa (chrysalis), and adult. The caterpillar looks nothing like the adult, and inside the pupa the body is rebuilt before the winged adult comes out. Scientists call insects that do this holometabolous.[4] Cockroaches grow the other way, by incomplete metamorphosis (called hemimetabolous). They skip the larva and the pupa. The nymph already resembles the adult, and it changes a little at each molt instead of being remade all at once.[4][5] Because there is no pupa, there is no single moment when the body is taken apart and put back together; the adult shape is reached gradually over the nymphal molts.[4] This is the normal pattern for cockroaches as a group, and studies of the cockroach Blattella germanica describe the same egg to nymph to adult sequence with no pupal stage.[5] Researchers think complete metamorphosis actually evolved from this simpler, hemimetabolous kind, so the hisser shows the older of the two patterns.[4]
What changes at each molt?
At each molt the nymph sheds its hard outer skin (the cuticle), expands, and hardens a new, larger one. In hissing cockroaches the wing pads, small flaps on the back, grow a little longer at every molt, and the dark pattern on the back and the size of two pale spots also shift step by step as the nymph ages.[6] These steady changes are why each of the six instars looks slightly different, and a keeper or student can roughly tell a nymph's stage by its size and how developed its markings and pads are.[6][3] Hormones set the pace. In cockroaches and other insects, a chemical called juvenile hormone keeps the animal in the nymphal state: it switches on a gene called Kr-h1 (through a receptor called Met), and Kr-h1 holds back an adult-making gene called E93.[5] As long as juvenile hormone stays high, each molt just makes a bigger nymph rather than an adult.[4]
Why is the contrast useful in the classroom?
Most life-cycle lessons show only the butterfly, so students can come away thinking every insect makes a cocoon. A live hisser is a clear counter-example. Its nymphs already look like the adult, so a class can watch a young roach grow up in plain sight, with shed skins as evidence of each molt, instead of a hidden change inside a chrysalis. Putting a hisser tank next to a butterfly poster lets students compare the two great patterns of insect growth side by side, and it makes the key idea easy to state in their own words: incomplete metamorphosis has no caterpillar and no pupa, while complete metamorphosis does. That fits common life-science units on animal life cycles and on how living things grow and change.
Open questions
Do the hisser's hormone genes work the same way as in other cockroaches?
The switch that ends the nymphal state is known, but mostly from other insects, not from the hisser itself. When juvenile hormone falls in the last nymphal stage, the adult-making gene E93 turns on; in the cockroach Blattella germanica, blocking E93 makes nymphs keep molting and even produces adults that can molt again, which shows E93 is the trigger for the final, adult molt.[7] A related gene, chinmo, holds the nymphal state in place and is switched off as the adult transition nears.[5][6] What is missing is direct work in Gromphadorhina portentosa. No one has shown that juvenile hormone, Kr-h1, and E93 act on the same schedule in this species. Settling it would take measuring these genes at each instar in the hisser itself instead of assuming the pattern carries over from Blattella.
Is the number of nymphal instars fixed at six?
Published work reports six nymphal instars for the hisser, with no extra or skipped stages noted.[3][1] But that count rests on a small number of reared animals, and no study has checked whether the number holds steady when temperature, diet, or crowding change, or whether males and females take the same number of molts. In other insects instar number can shift with rearing conditions, so the hisser's six may be a typical value rather than a fixed one. Rearing many hissers under controlled, varied conditions and counting their molts would settle it.
What environmental cues set the timing of the final molt?
The whole trip from hatching to a breeding adult takes about five months, but it is not clear what cues decide exactly when the last molt happens. Temperature, day length, food, and population density all affect development in other cockroaches, yet none of these has been tested as a trigger for the adult molt in the hisser. Controlled experiments that change one cue at a time and record when nymphs reach adulthood would show which signals matter and how much.
References
- Triet LM, Truong Thinh N (2025). Mitigating neural habituation in insect bio-bots: a dual-timescale adaptive control approach. Biomimetics (Basel). PubMed
- Monahan CF, Bogan JE Jr, LaDouceur EEB (2023). Histological findings in captive Madagascar hissing cockroaches (Gromphadorhina portentosa) and a literature review. Veterinary Pathology. PubMed
- Yoder JA, Glenn BD, Benoit JB, Zettler LW (2008). The giant Madagascar hissing-cockroach (Gromphadorhina portentosa) as a source of antagonistic moulds. Mycoses. PubMed
- Truman JW (2019). The evolution of insect metamorphosis. Current Biology. PubMed
- Escudero J, Gonzalvo J, Piulachs MD, Belles X (2025). Chinmo function in cockroaches provides new insights into the regulation and evolution of insect metamorphosis. PLoS Genetics. PubMed
- Nagata H, Suzuki Y (2026). Evolution of complete metamorphosis through temporal shifts in chronologically inappropriate morphogenesis (chinmo) and broad. Development. PubMed
- Kamsoi O, Belles X (2020). E93-depleted adult insects preserve the prothoracic gland and molt again. Development. PubMed
This deep dive backs the "Breeding & life cycle" section of the care guide.
Short, cited reads from the lab.