The supraesophageal ganglion (insect or fish brain) is the first part of the insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

 and fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 central nervous system

Nervous system

The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. It receives and processes information from the first, second, and third metameres. The supraesophageal ganglion lies dorsal to the esophagus and consists of three parts: the protocerebrum (with eyes), the deutocerebrum (with antennae in insect), and the tritocerebrum. The lobes of the tritocerebrum split to circumvent the esophagus and begin the subesophageal ganglion

Subesophageal ganglion

The subesophageal ganglion of insects is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands and certain muscles....

. The subesophageal ganglion continues the nervous system and lies ventral to the esophagus. Finally, the segmental ganglia are found in each body segment

Segmentation (biology)

Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...

as a fused ganglion and they provide the segments with some autonomous control.