Advertisement The absolute fastest growing tarantulas may reach 3 or 4 inches a year from birth, but that is generally the exception and not the norm. Most take years to reach that size, even with continuous feeding and warmth. I've had scorpions haven't reached an inch a year after their birth, not even 50% of their adult size. Heck many insects grow quite slow as well (beetles etc). Meanwhile someone barges in with a 30 lb puppy that's 6 months old. Endo/ectotherm aside, why do insects and arachnids grow so damn slow? It's almost frustrating, because I love them and I love raising them. Thoughts? Theories? Good reads?