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Simple Summary: Fatty acids play essential roles in insect growth and development. The key fatty acid synthesis reaction is catalyzed by fatty acid synthase (FAS). As a cofactor of fatty acid synthesis, however, little is known about the functions of acyl carrier protein (ACP) in insects and how it modulates the composition of fatty acids. We isolated an ACP from the black soldier fly Hermetia illucens (Diptera: Stratiomyidae) and investigated the expression patterns as well as functions in fatty acid synthesis and growth of insects. HiACP has a classical conserved region of DSLD. Based on the RNAi approach, we found that HiACP could regulate the fatty acid synthesis, in particular, reducing the composition of saturated fatty acids. Meanwhile, HiACP could also affect the growth and mortality of H. illucens. Acyl carrier protein (ACP) is an acyl carrier in fatty acid synthesis and is an important cofactor of fatty acid synthetase. Little is known about ACP in insects and how this protein may modulate the composition and storage of fatty acids. We used an RNAi-assisted strategy to study the potential function of ACP in Hermetia illucens (Diptera: Stratiomyidae). We identified a HiACP gene with a cDNA length of 501 bp and a classical conserved region of DSLD. This gene was highly expressed in the egg and late larval instars and was most abundant in the midgut and fat bodies of larvae. Injection of dsACP significantly inhibited the expression level of HiACP and further regulated the fatty acid synthesis in treated H. illucens larvae. The composition of saturated fatty acids was reduced, and the percentage of unsaturated fatty acids (UFAs) was increased. After interfering with HiACP, the cumulative mortality of H. illucens increased to 68.00% (p < 0.05). H. illucens growth was greatly influenced. The development duration increased to 5.5 days, the average final body weights of larvae and pupae were decreased by 44.85 mg and 14.59 mg, respectively, and the average body lengths of larvae and pupae were significantly shortened by 3.09 mm and 3.82 mm, respectively. The adult eclosion rate and the oviposition of adult females were also severely influenced. These results demonstrated that HiACP regulates fatty acid content and influences multiple biological processes of H. illucens. [ABSTRACT FROM AUTHOR]