Instar Stages

Instar stages are the developmental periods between successive molts in an invertebrate's life, and understanding them is fundamental to tracking how your captive-bred offspring are progressing from hatchling to adult. Every time an invertebrate molts, it enters a new instar, shedding its old exoskeleton to reveal a larger one underneath. The first instar is the stage immediately after hatching or birth, and subsequent instars are numbered sequentially until the animal reaches adulthood. For breeders, knowing what instar your animals are in tells you how far they are from maturity, what size prey to offer, when to expect the next molt, and when they might be ready to sell or rehome.

This topic applies to virtually all arthropod groups kept in the hobby, including tarantulas, scorpions, mantids, beetles, roaches, stick insects, centipedes, millipedes, and crustaceans like isopods and shrimp. The number of instars varies enormously between species, from as few as five or six in some mantids to dozens in large tarantula species that take years to mature. Mollusks do not molt and therefore do not have instars, but they do have recognizable growth stages that serve a similar tracking purpose.

Tracking instar stages matters for breeding because it gives you a developmental roadmap for each animal. Knowing that a tarantula sling is at third instar tells you approximately how many molts remain before maturity, what feeding schedule is appropriate, and roughly when the animal might be ready for pairing. For species with distinct morphological changes between instars, like wing development in mantids or color shifts in certain beetles, instar tracking also helps you confirm that development is progressing normally.

New breeders often ask how to tell what instar an animal is in, whether all instars take the same amount of time, and what happens when an animal seems stuck between molts. These questions reflect the practical reality that instar tracking is not always straightforward, especially when you acquire animals without hatch date information or when growth rates vary between individuals in the same clutch.

This article explains what instars are, how they work across different invertebrate groups, practical approaches to tracking developmental progress in your breeding stock, and the common mistakes that lead to misidentifying stages or mismanaging animals at critical developmental transitions.

An instar is technically the physical form an invertebrate takes between two molts. When people say an animal is in its third instar, they mean it has molted twice since hatching and is currently in the body it will wear until its next molt. Each instar represents a size increase, and often subtle changes in proportions, coloration, or features that reflect the animal's progression toward its adult form. The final molt into adulthood is called the ultimate or imaginal molt in insects, after which no further molting occurs. Arachnids and crustaceans continue to molt as adults, but their juvenile instars still represent distinct developmental stages.

Biologically, molting is driven by hormones that trigger the formation of a new, larger exoskeleton beneath the existing one. The animal absorbs fluid to expand the new exoskeleton, splits the old one, and emerges in a soft, vulnerable state that hardens over hours or days depending on the species. The time between molts, called the intermolt period, is when actual growth in body mass occurs as the animal feeds and builds the resources needed for the next molt. Intermolt duration varies based on temperature, food availability, and the animal's own metabolic rate, which is why two siblings from the same clutch can be at different instars within a few months.

The number of instars a species goes through before reaching adulthood is loosely determined by genetics but influenced by environmental conditions. Most mantis species go through six to nine instars. Tarantulas may go through eight to fifteen or more depending on species and sex, with males typically maturing in fewer instars than females. Beetles pass through a larval stage with its own instars before pupation and adult emergence. Scorpions typically molt five to seven times before reaching maturity. These numbers are averages, and individual animals can sometimes add or skip instars depending on conditions.

Each instar has its own care requirements that shift as the animal grows. First and second instar juveniles are tiny and fragile, needing small prey, high humidity, and minimal disturbance. Middle instars are typically the most resilient, growing steadily and feeding well. Later instars approach adult size and may begin showing sexual characteristics that allow you to identify sex, which is critical information for breeding projects. Recognizing these transitions helps you adjust care appropriately and make decisions about which animals to retain for breeding versus which to sell.

Identifying what instar an animal is in can be straightforward or challenging depending on the species and whether you have records from hatching. If you raised the animal from the egg, counting molts from hatch gives you an exact instar number. If you acquired the animal without records, you estimate based on body size relative to known adult size, observable features like wing bud development in insects, and comparison with documented growth charts for the species. Experienced keepers develop an eye for this over time, but exact identification without records is often an educated guess.

From a breeding perspective, instar tracking serves the practical purpose of helping you determine when animals are approaching maturity and might be ready for pairing. Premature pairing attempts waste time at best and risk injury at worst, while missing the maturity window in short-lived species means losing breeding opportunities entirely. Knowing that your male mantis is at penultimate instar tells you to start conditioning the female now so both animals are ready at the right time.

Tarantula instar stages are among the most frequently tracked in the hobby because the animals are long-lived and each instar represents months of growth. Slings emerge from the egg sac at first instar, which in many species is a pale, undeveloped form that does not feed. After their first molt into second instar, they begin eating and growing. Subsequent instars bring gradual increases in size and the slow development of adult coloration. Male tarantulas typically mature faster and in fewer instars than females, sometimes reaching their ultimate molt one to two years before females of the same species and clutch. Scorpion development follows a similar pattern, with first instars riding on the mother's back and subsequent instars growing independently.

Insect instar tracking involves distinct considerations depending on whether the species undergoes complete or incomplete metamorphosis. Mantids and stick insects undergo incomplete metamorphosis, meaning juveniles resemble small adults and add features like wings through successive instars. Tracking mantis instars is relatively straightforward because wing bud development provides clear visual markers of progress. Beetles undergo complete metamorphosis, meaning their larval instars look nothing like adults, and development includes a pupal stage between the final larval instar and the adult beetle. Roach instars in species like dubia follow incomplete metamorphosis, with nymphs gradually developing wing pads that indicate approaching maturity.

Myriapod instars are less commonly tracked in detail, partly because the animals are more difficult to observe closely without disturbance. Millipedes add body segments with each molt, which provides a rough indicator of developmental stage. Centipedes similarly add segments, though some species hatch with their full complement of legs and simply grow larger through successive molts. Both groups tend to have longer intermolt periods than insects, and exact instar counts are less well documented for many species in the hobby.

Crustacean development varies considerably. Isopods go through several mancae stages in the marsupium before release, then continue molting as juveniles. Freshwater shrimp hatch as miniature adults and molt frequently during rapid early growth. Crayfish juveniles molt multiple times in their first months and growth rate is strongly temperature-dependent. For all crustacean groups, water quality and mineral availability directly affect molting success, making environmental management during juvenile instars especially important.

Across all groups, the key principle is that instar stages provide a framework for understanding where an animal is in its development. The specifics differ enormously, but the concept of tracking molt-based progression applies universally to arthropod breeders.

The most reliable way to track instar stages is to start counting from hatch and record every observed molt. Keep a simple log for each animal or batch noting the hatch date, each molt date, and the current instar number. This data accumulates into a growth timeline that helps you predict future molts, identify animals developing faster or slower than average, and determine when individuals are approaching breeding maturity. For animals acquired without records, note the date you received them, estimate their current instar based on size and features, and begin tracking from that point.

Adjusting care based on instar stage is one of the practical benefits of tracking development. Early instar juveniles need smaller prey, higher humidity, smaller enclosures, and more frequent monitoring than later instars. As animals grow, you can increase prey size, expand enclosure dimensions, and reduce the frequency of humidity maintenance. The transition from juvenile to subadult instars is typically when you upgrade housing from small deli cups to intermediate enclosures, and when feeding frequency may shift from every few days to once a week or less for slower-growing species.

Recognizing premolt behavior helps you anticipate and support transitions between instars. Common premolt indicators include reduced or refused feeding, darkening of the exoskeleton in some species, increased hiding, and creation of a molt mat or web in arachnids. When you notice premolt signs, stop offering live prey that could disturb or injure a molting animal, ensure humidity is adequate to support successful ecdysis, and avoid handling or moving the enclosure. A failed molt is one of the most common causes of juvenile mortality, and supporting the process by maintaining good conditions and minimizing disturbance prevents many losses.

Using instar data to time breeding decisions is where developmental tracking pays off most directly. For species with short adult lifespans like mantids, knowing that a male is at penultimate instar tells you to begin conditioning the female for breeding so both animals reach maturity at compatible times. For tarantulas, knowing that a female is several instars from maturity helps you decide whether to hold a mature male or look for a pairing partner elsewhere. The more accurately you track development, the better you can coordinate breeding timelines.

Sharing instar information when selling or trading juveniles is good practice that helps buyers provide appropriate care. Listing an animal as a confirmed third instar with a known hatch date gives the buyer useful context for sizing enclosures, choosing prey, and estimating time to maturity. This transparency builds trust in the breeding community and helps your offspring receive proper care after they leave your hands.

The most common mistake with instar tracking is simply not doing it. Breeders who do not record molt dates lose the ability to predict development timelines, identify growth problems, and time breeding decisions accurately. Starting a tracking system requires minimal effort, just a notebook or spreadsheet with dates and instar numbers, but the information it provides over months and years of breeding is invaluable. Every molt you fail to record is data you cannot recover later.

Misidentifying instar stages by relying on size alone rather than actual molt counts leads to incorrect care decisions and premature breeding attempts. Size varies significantly between individuals of the same species and instar due to differences in feeding, temperature, and genetics. A well-fed third instar sling may be larger than an underfed fourth instar sibling. Using size as the sole indicator of development stage produces unreliable estimates that can lead to offering prey that is too large, attempting pairings too early, or mispricing animals for sale.

Ignoring premolt signs and continuing to offer live prey is a mistake that causes avoidable deaths during what should be routine molts. A cricket or roach left in an enclosure with a molting juvenile can injure or kill the animal while it is soft and defenseless. Learning to recognize the premolt cues for your species and pulling prey before the molt begins is a simple practice that saves lives. If you are not sure whether an animal is in premolt, remove prey as a precaution and check back in a few days.

Expecting all individuals from the same clutch to develop at the same rate is unrealistic and leads to frustration or worry when some animals lag behind siblings. Environmental variation in temperature across your shelf, individual genetic differences, and feeding competition if any communal rearing occurred all produce variation in growth rates. A sling that is one or two instars behind its clutchmates is not necessarily sick or stunted. It may simply be developing at the slower end of normal variation.

Failing to adjust care as animals progress through instars results in suboptimal conditions that slow growth or cause problems. Keeping a fifth instar tarantula in the same tiny deli cup it occupied as a second instar restricts movement and makes humidity management difficult. Continuing to offer pinhead crickets to an animal that has graduated to small roaches wastes feeding opportunities and slows growth. Reassess your care setup every few instars and make adjustments that match the animal's current size and developmental needs.

Instar stages give you a developmental roadmap for every arthropod you breed, and tracking them turns guesswork into informed husbandry decisions. Knowing what instar your animals are in helps you size prey correctly, time enclosure upgrades, anticipate premolt periods, and determine when individuals are approaching breeding maturity. The tracking itself is simple, requiring only a dated record of each observed molt, but the value it provides compounds over the lifetime of each animal and across your breeding program as a whole.

Responsible breeders use instar information to make better decisions about pairing timing, sale readiness, and care adjustments that directly affect animal welfare. Selling a juvenile with accurate instar and hatch date information helps the buyer provide appropriate care, which means your offspring are more likely to thrive after leaving your collection. This kind of transparency strengthens the breeding community and reflects well on breeders who take the time to maintain good records.

Species-specific knowledge about instar counts, intermolt duration, and developmental milestones is essential because the details vary so much across invertebrate groups. A mantis that goes through seven instars in four months is on a completely different timeline than a tarantula that goes through twelve instars over several years. Research the expected developmental trajectory for your specific species so you know what normal looks like and can recognize deviations early.

Tracking instars connects you more closely to the animals you breed by giving you a window into their development that casual observation does not provide. Each molt is a milestone, each instar brings visible changes, and the progression from tiny hatchling to mature adult is something you can follow in detail when you keep good records. That connection makes the work of breeding more meaningful and helps you raise healthier, better-documented animals that reflect the care and attention you invested from the very first instar.