Length up to 2.5 cm, with four apple-green spots on its black back, the four-spot midget is a very distinctive and attractive damselfly`1. Females may be one of two forms, and whilst some are like the previous description, others are orange. Immature four-spot midgets are greyish but still have four pale spots visible on the back

Unlike many damselflies, the four-spot midget does not leave the site in which they emerged, and instead will stay in the reed community where they will spend their entire adult life. Adult four-spot midgets can be seen flying about their habitat from late May to early August. Like the larvae, the adults are generalized, opportunistic feeders, which prey on smaller flying insects, such as midges and mosquitoes. The four-spot midget is a weak flier, and adopts a sit-and-wait tactic when searching for prey or mates. Its weak flying abilities, along with its small body size and bright colouration, especially of the males, may mean it is particularly vulnerable to predation. The average lifespan of this damselfly is estimated to be around 35 days

The majority of the populations are associated with brackish marshes, but populations have also been found in coastal freshwater streams. In Hong Kong the adults have been recorded in mangrove habitat, but the larvae have never been found there. Larvae of Mortonagrion hirosei have a relatively high survival rate in brackish water of saline concentrations of up to 15%. 

Occurs in Japan, Hong Kong and Taiwan. It may also exist in mainland China. The species is confined to the coastal regions of Hong Kong, Taiwan, and the east coast of Honshu, Japan. There is a single record from an inland site in Gutoushan, Guangdong, China.

The four-spot midget is classified as Near Threatened (NT) on the IUCN Red List. This vulnerable insect is threatened by habitat loss and degradation (1). In Japan, the extent of reedbeds has been substantially reduced by river alterations and the destruction of wetlands, and suitable habitat in mainland China is similarly being lost to city development, reclamation and dam construction