Symbolism, Imagery, Allegory
It seems Raveloe is out of fly swatters, since the village is infested with bugs. In Silas Marner, insects, particularly spiders, appear all the time. Okay, not literal insects, but metaphorical ones.
Silas, the narrator says, "seemed to weave, like the spider, from pure impulse, without reflection" (1.2.3); he sits at his loom with "his eyes bent close down on the slow growth of sameness in the brownish web" (1.2.10); Eppie calls him away from "the repetition of his web" (1.14.33). Spiders represent all that is inhuman about work. When work is done without thought or love, it's mechanical, and mechanical work is insect labor.
Spiders also let Eliot take a jab at industrialization. By the 1860s, when Eliot wrote Silas Marner, almost all weaving and spinning was done by machine. The kind of work that people do at machines is what Karl Marx in 1844 called "alienated" or "estranged" labor—work that makes people unable to control their own destinies.
Farmers, for example, work for themselves (mostly). Their labor directly produces the food that sustains them. Artisans produce things. They make chairs, clothes, candles, wheels, one-of-a-kind products that they make from start to finish. But someone working at a machine only makes part of something, or makes a product in which he or she has no investment. To Eliot, that kind of work is insect-labor. It's dehumanizing.