Fig. 4. The rectum of a larva that had ingested azolitmin in vivo, reared and assayed in alkaline water (pH 11). Azolitmin is blue in alkaline water (pH >6.2), and red in acidic water (pH <6.2). A total of 36 larvae were assayed (12 larvae each in RS 4, RS 7 and RS 11). The rectal contents were red (pH <6.2) in all larvae. Top, the original image. Bottom, the same image adjusted for brightness (+39) and contrast (+35) using Adobe Photoshop.