Insect brain in the context of "Segmentation (biology)"

Insects (from Latin insectum) are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes, and a pair of antennae. Insects are the most diverse group of animals, with more than a million described species; they represent more than half of all animal species.

The insect nervous system consists of a brain and a ventral nerve cord. Most insects reproduce by laying eggs. Insects breathe air through a system of paired openings along their sides, connected to small tubes that take air directly to the tissues. The blood therefore does not carry oxygen; it is only partly contained in vessels, and some circulates in an open hemocoel. Insect vision is mainly through their compound eyes, with additional small ocelli. Many insects can hear, using tympanal organs, which may be on the legs or other parts of the body. Their sense of smell is via receptors, usually on the antennae and the mouthparts.

Insect cognition describes the mental capacities and study of those capacities in insects. The field developed from comparative psychology where early studies focused more on animal behavior. Researchers have examined insect cognition in bees, fruit flies, and wasps.  

Research questions consist of experiments aimed to evaluate insects' abilities such as perception, emotions attention, memory (wasp multiple nest), spatial cognition, tools use, problem solving, and concepts. Unlike in animal behavior the concept of group cognition plays a big part in insect studies. It is hypothesized some insect classes like ants and bees think with a group cognition to function within their societies; more recent studies show that individual cognition exists and plays a role in overall group cognitive task.

Prothoracicotropic hormone (PTTH) was the first insect hormone to be discovered. It was originally described simply as "brain hormone" by early workers such as Stefan Kopeć (1922) and Vincent Wigglesworth (1934), who realized that ligation of the head of immature insects could prevent molting or pupation of the body region excluded from the head if the ligation was performed before a critical age in the lifestage was reached. After a certain point the ligation had no effect and both sections of the insect would molt or pupate. However, implantation of a conspecific brain to a sessile ligated abdomen or an abdomen under diapause would induce molting or pupation. Thus, the brain was originally thought to be the source of the hormone that induces molting in insects.

Later it was established that the insect brain produces a number of hormones, but the hormone which was the cause of the observations made by Kopeć and Wigglesworth was prothoracicotropic hormone. PTTH is secreted by a neurohemal organ, the corpus cardiacum (in some insects the corpus allatum secretes PTTH) which is actually a discrete structure posterior to the brain. PTTH is released in response to environmental stimuli and as its name implies PTTH acts on the prothoracic glands, which respond by releasing molting hormone (an ecdysteroid) into the haemolymph. Molting hormone stimulates the molting process.