Both spiders and scorpions have distinct sexes, except for a few species which reproduce by parthenogenesis and have no males. In spiders there is often a clear sexual dimorphism (males differ from females), in contrast to scorpions. Thus it is not easy to determine the sex of a scorpion. Immature scorpions all look like females. Adult males tend to have a relatively larger metasoma and pedipalps. Sometimes there is some allometric growth (changed proportions) in the pedipalps of the males, so that they are heavier, longer or thicker than those of the females. The pecten of the male is usually somewhat larger and has more and larger teeth. During the reproductive period a male scorpion deposits a stalked spermatophore on a smooth firm spot on the ground. This is a packet of gelatin that has a very specific form with a droplet of sperm on top. The male grips the female and together they engage in a mating dance ("pas de deux"). Some species intertwine their tails during courtship. During this mating dance he leads the female over his packet of sperm, whereby fertilisation occurs. The females do not lay eggs, but instead give birth to live young: small white, developed mini-scorpions, only a few mm in size. Many scorpions are subsocial, meaning that a female carries her young on her back until their first moult (at around one to four weeks). During this period the little ones still feed on the yolk remains in their body. Afterwards the creatures choose to live in isolation. They are active at night. They shun light and hide during the day in anything lying on the ground, under stones or wood, in footwear, clothes or blankets. Some species burrow underground. The animals are sexually mature after 6 months to 3 years, following four to seven moults.