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Description
Mating millipede forming a "Heart" shape. Taken at night in Singapore.

Quote from en.wikipedia.org/wiki/Milliped…
Millipedes show a diversity of mating styles and structures. In the basal order Polyxenida (bristle millipedes), mating is indirect: males deposit spermatophores onto webs they secrete with special glands, and the spermatophores are subsequently picked up by females.[14] In all other millipede groups, males possess one or two pairs of modified legs called gonopods which are used to transfer sperm to the female during copulation. The location of the gonopods differs between groups: in males of the Pentazonia they are located at the rear of the body and known as telopods and may also function in grasping females, while in the Helminthomorpha – the vast majority of species – they are located on the seventh body segment.[8] A few species are parthenogenetic, having few, if any, males.[15]

Gonopods occur in a diversity of shapes and sizes, and in the range from closely resembling walking legs to complex structures quite unlike legs at all. In some groups the gonopods are kept retracted within the body, while in others they project forward parallel to the body. Gonopod morphology is the predominant means of determining species among millipedes: the structures may differ greatly between closely related species but very little within a species.[16] The gonopods develop gradually from walking legs through successive moults until reproductive maturity.[17]
Growth stages of Nemasoma (Nemasomatidae), which reaches reproductive maturity in stage V

The genital openings (gonopores) of both sexes are located on the underside of the third body segment (near the second pair of legs) and may be accompanied in the male by one or two penes which deposit the sperm packets onto the gonopods. In the female, the genital pores open into paired small sacs called cyphopods or vulvae, which are covered by a small hood-like cover, and are used to store the sperm after copulation.[10] The cyphopod morphology can also be used to identify species. Millipede sperm is aflagellate (lacks a flagellum), a unique trait among myriapods.[8]

In all millipedes except the bristle millipedes, copulation occurs with the two individuals facing one another. Copulation may be preceded by male behaviors such as tapping with antennae, running along the back of the female, offering glandular secretions which the female consumes, or in the case of some pill-millipedes, stridulation or "chirping".[18] During copulation in most millipedes, the male positions his seventh segment in front of the female's third segment, and may insert his gonopods to extrude the vulvae before bending his body to deposit sperm onto his gonopods and reinserting the "charged" gonopods into the female.[13]

Females lay between ten and three hundred eggs at a time, depending on species, fertilising them with the stored sperm as they do so. Many species simply deposit the eggs on moist soil or organic detritus, but some construct nests lined with dried faeces, and may protect the eggs within silk cocoons.[10] In most species the female abandon the eggs after laying but some species in the orders Platydesmida and Stemmiulida provide parental care for eggs and young.[14]

The young hatch after a few weeks, and typically have only three pairs of legs, followed by up to four legless segments. As they grow, they continually moult, adding further segments and legs as they do so. Some species moult within specially prepared chambers of soil or silk,[19] which they may also use to wait out dry weather, and most species eat the shed exoskeleton after moulting. The adult stage- when individuals become reproductively mature- is generally reached in the final molt stage, which varies between species and orders, although some species continue to molt after adulthood. Furthermore, some species alternate between reproductive and non-reproductive stages after maturity, a phenomenon known as periodomorphosis, in which the reproductive structures regress during non-reproductive stages.[15] Millipedes may live from one to ten years, depending on species

Description
Mating millipede forming a "Heart" shape. Taken at night in Singapore.

Quote from en.wikipedia.org/wiki/Milliped…
Millipedes show a diversity of mating styles and structures. In the basal order Polyxenida (bristle millipedes), mating is indirect: males deposit spermatophores onto webs they secrete with special glands, and the spermatophores are subsequently picked up by females.[14] In all other millipede groups, males possess one or two pairs of modified legs called gonopods which are used to transfer sperm to the female during copulation. The location of the gonopods differs between groups: in males of the Pentazonia they are located at the rear of the body and known as telopods and may also function in grasping females, while in the Helminthomorpha – the vast majority of species – they are located on the seventh body segment.[8] A few species are parthenogenetic, having few, if any, males.[15]

Gonopods occur in a diversity of shapes and sizes, and in the range from closely resembling walking legs to complex structures quite unlike legs at all. In some groups the gonopods are kept retracted within the body, while in others they project forward parallel to the body. Gonopod morphology is the predominant means of determining species among millipedes: the structures may differ greatly between closely related species but very little within a species.[16] The gonopods develop gradually from walking legs through successive moults until reproductive maturity.[17]
Growth stages of Nemasoma (Nemasomatidae), which reaches reproductive maturity in stage V

The genital openings (gonopores) of both sexes are located on the underside of the third body segment (near the second pair of legs) and may be accompanied in the male by one or two penes which deposit the sperm packets onto the gonopods. In the female, the genital pores open into paired small sacs called cyphopods or vulvae, which are covered by a small hood-like cover, and are used to store the sperm after copulation.[10] The cyphopod morphology can also be used to identify species. Millipede sperm is aflagellate (lacks a flagellum), a unique trait among myriapods.[8]

In all millipedes except the bristle millipedes, copulation occurs with the two individuals facing one another. Copulation may be preceded by male behaviors such as tapping with antennae, running along the back of the female, offering glandular secretions which the female consumes, or in the case of some pill-millipedes, stridulation or "chirping".[18] During copulation in most millipedes, the male positions his seventh segment in front of the female's third segment, and may insert his gonopods to extrude the vulvae before bending his body to deposit sperm onto his gonopods and reinserting the "charged" gonopods into the female.[13]

Females lay between ten and three hundred eggs at a time, depending on species, fertilising them with the stored sperm as they do so. Many species simply deposit the eggs on moist soil or organic detritus, but some construct nests lined with dried faeces, and may protect the eggs within silk cocoons.[10] In most species the female abandon the eggs after laying but some species in the orders Platydesmida and Stemmiulida provide parental care for eggs and young.[14]

The young hatch after a few weeks, and typically have only three pairs of legs, followed by up to four legless segments. As they grow, they continually moult, adding further segments and legs as they do so. Some species moult within specially prepared chambers of soil or silk,[19] which they may also use to wait out dry weather, and most species eat the shed exoskeleton after moulting. The adult stage- when individuals become reproductively mature- is generally reached in the final molt stage, which varies between species and orders, although some species continue to molt after adulthood. Furthermore, some species alternate between reproductive and non-reproductive stages after maturity, a phenomenon known as periodomorphosis, in which the reproductive structures regress during non-reproductive stages.[15] Millipedes may live from one to ten years, depending on species