Fertilization is internal; however, exchange of sperm occurs in a variety of fashions. Sexual receptivity is associated with adult molting and in some species pheromones to facilitate aggregation of sexes. The sperm is produced in a packet, often with a stalk holding it above the substrate. In some groups (most Onychiuridae) these packets are produced randomly and fertilization occurs by accidental contact of the female with the packet of sperm. In a number of species the packets are produced only in the presence of females, but the most elaborate procedures are seen in Podura aquatica and the Sminthuridae. Here, often, there are elaborate courtship and maneuvering associated with fertilization. This is often accompanied by modifications in male anatomy, which ensure the appropriate species response and/or positioning for sperm packet uptake. Most of these species are brightly colored and patterned, which may also be associated with species recognition. In these forms, sexual dimorphism is the rule and often extreme. This is also true of many marine littoral species, but in these, the method of sperm transfer is still unknown and the function of the dimorphic structures (usually male) is unclear.
In some members of the family Isotomidae secondary sexual characters alternate with molts, being expressed in stages in which the animals are sexually receptive and not expressed in stages in which they are not receptive. In most Collembola there is little or no sexual dimorphism and sexes can be separated only by the difference in their genital openings. Both males and females occur in most species but parthenogenesis is common, especially in some genera of the Tullbergiidae.
Development is direct, with the young generally very similar to the adults except for the absence of sexually associated features and body ratios and some aspects of the setae clothing. The main exception to this generalization is in the Tomoceridae, whose juveniles have been assigned to genera different from those of the adults. Collembola continue to molt after reaching sexual maturity and some species can molt very large numbers of times (the record is 52). They stop reproducing at some point and later molts result in reduced rather than increased size. Although some Collembola have been known to live more than 5 years in captivity, their life span in the wild is undoubtedly much shorter.
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