Abstract
There are, in relation to the number of ganglia, three main types of ventral nerve cords in the adult stingless bees: 1) Those which have two ganglia in the thorax and five in the abdomen, 2) those which have two ganglia in the thorax and four in the abdomen, and 3) those which have three ganglia in the thorax and four in the abdomen. Intermediate types between those listed exist.
The most generalized ventral nerve card was found in the workers of Trigona fulviventris, where each ganglion lies only one segment ahead from its respective segment (Fig. 1, A). When this generalized type of nerve cord is taken as a starting point, there are two major recognizable evolutionary trends almong the stingless bees (Fig. 1). One trend is toward the fusion of ganglion 7 with the preceding abdominal ganglion. This occurs independently in the several subgenera of Trigona, with a series of intermediate steps (Fig. 3, A to H). Among some of the species of Melipona there is also a tendency of fusion of ganglion 7 with ganglion 6 (Fig. 4, A to D ) but none of the species of Mélipona dissected was found with those ganglia completely fused.
The second trend noted is toward cephalization of the nerve cord ( Fig. 2). This occurs only in Melipona. Here the abdominal ganglia migrate from two to four segments ahead from their respective segments. As a result, ganglion 3, which in primitive forms was located in the second abdominal segment (first metasomal segment), has moved well inside the thorax. The presence of ganglion 3 in the thorax is characteristic of all the species of Melipona. The highest type of cephalization is found only in a few species, such as MeliPona flavipennis in which ganglia 4 and 5 have moved into the second abdominal segment, and ganglia 6 and 7 into the third. Therefore, in this type, the card extends in the abdomen only as far as the third abdominal segment (second metasomal segment, Fig. 2, E ) . There are, of course, intermediate types.