The Fly

At a meet-the-press event sponsored by Bartok Science Industries, Seth Brundle, a brilliant, eccentric scientist, meets Veronica Quaife, a science journalist. Seth takes Veronica back to the warehouse which as serves as both his home and laboratory, where he shows her a project that will change the world: a set of "Telepods" that allows instantaneous teleportation of an object from one pod to another. Brundle convinces Veronica to keep the project's existence quiet in exchange for exclusive rights to the story, and she begins to document his work. Although the Telepods can transport inanimate objects, they cannot teleport living things, a test baboon is turned-inside out during an experiment.
Seth and Veronica soon begin a romantic relationship, and their first sexual encounter provides inspiration for Seth, who reprograms the Telepod computer to cope with living flesh. Shortly thereafter, he successfully teleports a second baboon. Seth wants to have a romantic celebration with Veronica, but she abruptly departs without telling him why. Seth's judgment soon becomes impaired by alcohol and his fear that Veronica is secretly rekindling her relationship with her editor and former lover, Stathis Borans. In reality, Veronica has left to confront Borans about a veiled threat, spurred by his romantic jealousy of Brundle, to publish the Telepod story without her consent. Drunk and jealous, Seth hastily teleports himself, unaware that a common housefly has slipped inside the transmitter pod. Brundle emerges from the receiving pod, seemingly normal.
After Seth and Veronica reconcile, Seth exhibits what at first appear to be beneficial effects, such as increased strength, stamina, and sexual potency, and he mistakenly believes that the teleporter has improved and "purified" his body. Veronica, however, is more concerned about Seth's growing mania and the strange, bristly hairs growing out of a recent wound on his back. Brundle quickly becomes unstable, intolerant and violent, and he tries to force Veronica to undergo teleportation. When she refuses, he abandons her to indulge in a barroom arm wrestling match and casual sex with a woman named Tawny, whom he picks up at the bar. Veronica's next-morning arrival at the lab spares Tawny from being forcibly teleported. Brundle angrily throws Veronica out and dismisses her concerns about his health, but when his fingernails begin falling off, he realizes something went horribly wrong. He checks his computer's records and discovers the Telepod computer, confused by the presence of a secondary life-form, merged him with the fly at the genetic level.
Four weeks later, Seth reaches out to Veronica, and theorizes that he is slowly becoming a hybrid creature, which Seth dubs "Brundlefly". Now severely disfigured, Brundle exhibits fly-like characteristics, such as vomiting digestive enzymes onto his food, and the ability to cling to walls. He also realizes that his motives and compassion as a human are waning, replaced by savage impulses. Attempting to find a cure for his condition, Brundle installs a fusion program into the Telepod computer to dilute the fly genes in his body with more human DNA. To her horror, Veronica learns that she is pregnant by Seth, and cannot be sure when the child was conceived. After she has a nightmare of giving birth to a giant maggot, a terrified Veronica seeks a late-night abortion, only to be abducted by Brundle, who begs her to carry the child to term, since it could be the last remnant of his untainted humanity. Veronica sadly refuses, afraid that the child will be a hideous mutant. When Borans, armed with a shotgun, attempts to rescue Veronica, the horribly-degenerated Brundle dissolves Borans' hand and foot with his corrosive enzyme.
Brundle then reveals his desperate, last-ditch plan to Veronicaâhe will use the three Telepods (the third pod being the prototype) to fuse himself, Veronica, and their unborn child together into one entity, so they can be the "ultimate family". Veronica frantically resists Brundle's efforts and accidentally tears off his jaw, triggering his final transformation into a monstrous hybrid. The "Brundlefly" creature traps Veronica inside Telepod 1 and steps into Telepod 2. However, the wounded Borans uses his shotgun to sever the cables connecting Veronica's Telepod to the computer, allowing Veronica to escape unharmed. Breaking out of its own pod just as the fusion process is activated, Brundlefly is gruesomely fused with the metal door and cabling of Telepod 2. As the mortally-wounded Brundlefly/Telepod creature crawls out of the receiving pod, it silently begs Veronica to end its suffering with Borans' shotgun. Veronica tearfully hesitates, then pulls the trigger.

Question: What makes Brundle look more and more like a fly?