Brachyura larvae
The megalopa

—Cephalothorax
The body of a megalopa is formed by an oval or rectangular cephalothorax (Megalopa dorsal view), externally composed of a dorsal carapace and a ventral sternum, plus a segmented abdomen. Large stalked pigmented eyes and other appendages emanate from the cephalothorax, as do swimming appendages from the abdomen. Both the cephalothorax and abdomen are pigmented in live or freshly preserved specimens. This is not shown in the figures.

The carapace frequently has a rostrum, except in Zaops. The carapace can de ornamented by protuberances and bear a posteriorly directed median spine (Parthenope :Parthenope(Platyl.)serrata 3). The five pereopods (p1, p2, p3, p4, p5) are fully formed, the first one being chelate (likewise also called cheliped) (Megalopa dorsal view).

The abdomen is composed of six somites in all megalopae studied, except in Zaops and Persephona, in which only five somites are present, or the 6th is incipiently formed. Ventrally the abdomen bears functional biramous pleopods on somites 2-5, and a pair of uniramous uropods on somite 6 in most species.

—Antennule
The antennule (Megalopal appendages 3) is developed as a stout 3-segmented peduncle bearing an endopod and exopod. The exopod segments are often armed with aesthetasc setae.

—Antenna
The antenna (Megalopal appendages 1) is composed of a peduncle (protopod) with 4 segments and a multisegmented flagellum (endopod).

—Mandible
The mandibles (Megalopal appendages 2) no longer have distinct molar and incisor portions but a cutting edge instead. There is a dorsal palp that usually bears setae.

—Maxillule and maxilla
The maxillule and maxilla (Megalopal appendages 2) have changed little since the zoeal stage, except that the endopod now is usually reduced.

—First maxilliped
The first maxilliped (Megalopal appendages 3) has a protopod from which arise broad coxal and basial segments. A narrow endopod and a 2-segmented exopod have developed. The outer margin of the protopod has an epipod with a variable number of plumodenticulate setae.

—Second maxilliped
The second maxilliped (Megalopal appendages 4) has a small protopod from which arises an endopod with four clearly demarcated segments and a 2-segmented exopod. For known species there is no epipod emanating from the protopod.

—Third maxilliped
The third maxilliped (Megalopal appendages 4) has a protopod from which arises a well developed 5-segmented endopod and a narrow 2-segmented exopod. The protopod is usually broad, with the external margin bearing a well developed epipod covered with plumodenticulate and other setae.

—Pereopods
The pereopods (Megalopa dorsal view) have seven segments in the majority of the brachyuran megalopae. However, the basial segment is not always clearly differentiated from the ischium. The lower margin of the dactyl on the 5th pereopod (p5) is armed with simple setae in almost all species studied.

—Pleopods
The pleopods (Megalopal appendages 1) are attached to the ventral surface of the abdominal somites (see, for instance, Aratus pisonii ov). Each appendage bears long marginal natatory plumose setae, the number of setae decreasing on succeeding pairs. A small endopod is present which is distally armed with a number of specialized setae that interlock with those of the adjacent pleopod and allow for synchronized movement of the swimming appendages.